I witnessed something similar on a PHX-IAH flight before Christmas last year. One GA was "randomly" selecting people and forcing them to pay to check their carry-ons. Now some of the carry-ons were in fact too large, but some were borderline and while the "cop" GA was busy processing the bag fees and tagging the bags the other GA allowed people to board, sometimes with bags larger than those that were carried by PAX that had been forced to check their bag.

I have no problems with GAs making people pay to check bags that have no business being carried on, but I do have a problem with selective enforcement such as I witnessed and I have a problem with what was described by the OP.