I hear what you're saying, and that makes sense. I'll take a look at some of my guys to see if A) they've shown a drop in fouls/ over the seasons, and
if it correlates to a specific skill increase. It might be a hard analysis, though, because for most of my guys they've been increasing in a variety of skills, and experience, the whole time. Plus, it seems like playing in a higher league would 'undo' some benefits from training since you're playing against tougher competition, so that will need to be taken into account.
And to add a useful post to the thread too, I also think sometimes improving defense can actually increase fouls - for example, if you increase a big man's OD a few levels, he may end up getting more off the ball fouls called on him because he thinks he can try to steal the ball, when before he couldn't.