That misconception doesn't just exist in the USA, I have heard of it since I was a kid, and is most likely a product of Hollywood.
In the country of New Zealand, there is no differentiation in law between a trained martial artist and the ordinary person on the street. You have no extra legal obligations than the normal person next to you. But much like a police officer and judge are both subject to the exact same laws as everyone else, like them you also have extra moral or social obligations that the average person may not be subject to. These obligations usually come into consideration after you have used your skills to defend yourself.
The law here can be summarized as saying:
You may use the minimum necessary force to defend yourself
In the past the school I train at has obtained a legal definition of exactly what this means, which is: as soon as I reasonably believe my safety to be in danger I may use reasonable force to protect myself. That reasonable belief can exist the moment I see my opponent telegraph a move, I can then use a reasonable amount of force to defuse that situation and preserve my safety. As a trained martial artist there is a far higher duty of care required in my application of force to my opponent, and this changes the definition of what can be considered reasonable.
If a situation ever occurs, being careful will serve you well after the fact. Consider what a jury would think during a trial on assault charges:
- the attacker has two broken limbs, three dislocated joints, and exhibits signs of having lost consciousness in a sudden way (i.e. lack of memory, possible concussion). The defendant (you) says that he honestly only used the minimum amount of force to defend himself. Was the attacker particularly tough and persistent and high on methamphetamine and was consequently tough to stop? Or did the defendant not stop when he should have and administered his own brand of justice?
- the attacker claims he did nothing and the defendant just hit him out of nowhere. The defendant said he saw the attacker start to attack so he (legally) acted first. The attacker's telegraphing moves are too small to be seen on the grainy CCTV footage shot from 20 feet away in dim lighting conditions. The people standing around either didn't see what happened or were too drunk to be reliable witnesses. Can the defendant really justify the dislocated shoulder or knee suffered by the attacker?
- the attacker is claiming vast amounts of monetary compensation for damages because although the attacker was drunk, hostile, belligerent, out of control and was threatening your girlfriend with forms of behaviour more at home in a fifteenth century torture chamber, you gave him a sore head and because you are a trained martial artist you should have known better and should have just humored him. Now he is suffering lifelong braincell loss from your blow and he didn't actually do those things to your girlfriend so he is the victim.
In all these cases you could have acted perfectly reasonably at the time, but who is the jury (or judge) going to believe? They weren't there, they didn't experience what happened, they are analyzing the event after the fact. Your extra responsibilities due to your training dictate that you only do what is absolutely necessary. So although you may have 50 lethal moves in your repertoire, you only use the minimum number of non-lethal moves necessary to defuse the situation.
A guilty or innocent verdict often comes down to interpretation of imprecise wording and interpretation of confusing and muddled events, and as a trained martial artist you may have a disadvantage due to people's perceptions of you and what you do.
Ironically, just over a year since I wrote that answer, I had to serve on a jury in an assault with a weapon case where the accused pleaded self defence, and this gives me some insight into how this can play out with respects to a martial artist.
The accused was not a martial artist, but there were many parts of point #2 from above present in the case:
- the complainant claimed he was attacked first, the accused said she was attacked first and defended herself
- grainy inconclusive CCTV footage shot from a distance
- unreliable eye witnesses
- excessive and/or random injuries suffered by the complainant that were out of scale compared to the alleged infringement
This shows how conflicting or contradictory the case can be when self defense is claimed. In this case the accused is innocent until proven guilty which means the prosecution have to prove that it wasn't self defense.
If the accused is a martial artist the prosecution's job is easier because the jury will have an automatic bias even though they don't mean to (remember that they are ordinary people from the community). This will be compunded by the fact that due to your training you may not remember the exact sequence of actions that took place and the exact force you may have hit with - you are trained to react without conscious thought which makes remembering really hard and the jury will not necessarily understand this. If the case relies upon balance of probabilities then it will be harder for you to convince the judge/jury that your actions were reasonable.
Conclusion: (in this jurisdiction) there are no specific legal ramifications for being a martial artist involved in a fight, but there will certainly be consequences or complications for you should it reach trial and you are the accused.