I agree, it's hard for referees to call a foul when a punch is already started and the other fighter moves into a position which causes or contributes to the foul.
Referees don't mind giving warnings but they dislike taking a point, that's a fact. You see a couple warnings for a foul plenty of times but the third and fourth foul seems to be real hard for the ref to see.
I can't remember the last time I saw a point deduction for the ubiquitous punch to the back of the head from the clinched position, maybe because when one fighter starts doing it the other retaliates in kind, usually.
The ref has a tough job, he's supposed to protect both fighters, make sure all the rules are followed and do it without becoming a big factor in the outcome. An overly aggressive referee can kill the flow of a fight and bring me to the point of yelling at the TV, a lax or partial ref can allow one fighter to take over a fight which usually has me yelling at the TV also. Refereeing has never been a job I would want.