I agree their contracts would have some vague clause about the club being able to deduct wages decided at the club's discretion: that is too uncertain for me. Whatever their contracts do say, I doubt some of those fines would be legally enforceable - they may be some players entire wages for a couple of weeks.I thought it's standard procedure for most clubs, except we don't always get to see the details.
Probably enforeceable via the terms in the player's contract.