People are in favor of offering donations and having the modder get the full donation, not being asked to pay for added content. This includes full-fledged mods to a single weapon skin. To make things worse, it seemed that the store became flooded with half-assed content as a means for modders to make a quick buck. Furthermore, some mods required other mods, if the mods that were required started to charge, it became EXTREMELY prohibitive to piggyback.

Most gamers are aware of how Valve became so big and it's through the modding community in HL1 and eventually HL2. Can you imagine how prohibitive it would have been for Counter-Strike to grow if they charged $10 for the mod (obviously when it was till a mod =< v1.6)?

On top of that, some mods were genuine improvements over the original. For instance, there is a huge Skyrim UI mod that is a vast improvement over the vanilla. Some people were actually afraid that game companies would use this pay for mod system as a way to double dip. "Let's release a half-assed UI, let some modder charge for his better UI and take 45-50% of his sales."