Casual players make illegal moves all the time. The problem isn't knowing how the pieces move. It's that it's illegal to leave your own king in check. It's not so common to accidentally move your king into check, though I'm sure it happens, but it's very common to accidentally move a piece that was blocking an attack on your king.

I would tend to agree that there's a big difference between attempting to make a move that's illegal because of the state of a different region of the board, and attempting to make one that's illegal because of the identity of the piece being moved, but if your only category of interest is "illegal moves", you can't see that difference.

Software that knows the rules of the game shouldn't be making either mistake.

Casual players don’t make illegal moves so often that you have to assign them a random move after 10 goes.