Because of several reasons, the primary being pointed out by others already... And I know, you're just saying it as rhetoric, but I feel I need to address it as though it was a serious suggestion in order to point out just how unhelpful and potentially even damaging your idle comment can be.
1) You're alienating a good portion of the population by essentially saying men, as a group, have a defining trait of harassing people/not knowing how to behave. This will put people on the defensive, and similar messages has already put many men and women off the idea of feminism. In some cases, it might even backfire and be the cause of further antagonism for petty reasons.
2) For such training to have any effect, the vast majority of perpetrators must be honestly ignorant of how vile their behavior is, and not just scummy individuals. Otherwise, the training will be disregarded.
3) Giving such training would take the above two aspects to their extreme conclusions AND add a huge financial, legislative and administrative burden to the training-giving organization, for it to have any sort of effect.
4) Under the assumption that it's not only men who perpetrate sexual harassment/misbehave on public transport, this would not solve the problem by any stretch of the imagination. Reduce? Depends on the radicalization-by-perceived-opposition the first point would cause.
What you would need to do, is point out how effed up the behavior in question is (It's shocking to me this is apparently required in a city like Paris), and treat that, rather than giving training to a group of people.
Focus on the behavior, not the people or risk creating your own enemies where there would be allies. Until you can reliably deal with the behavior, the least you can do is create a bigger barrier to committing such behavior by making it harder to do without repercussions. Making women capable of incapacitating such criminals until police arrives would certainly be the most effective way of making it more difficult for people to commit physical sexual harassment and get away with it WHILE allowing those women to feel more empowered/safer/confident in themselves.
The behavior definitely needs addressing, however. Shocking that it still does.