If you buy a new monster book, and put in a new monster, and it turns out to suck (overpowered, underpowered, just plain unfun), the DM can say they're sorry, they won't use that monster again, and move on.
When a player buys a new book (or even uses the CB) and uses some overpowered or underpowered or just plain unfun build, it's different. If they're underpowered, usually they just switch to something else. Unfun might be unfun for the rest of the group, creating conflict. And taking away something overpowered also sparks conflict between player and DM. I took away a player's Battlerager Vigor a session ago, and he dropped his fighter entirely and made a wizard instead. He always complains that his Battlerager wasn't that overpowered, wants to face an NPC version (which wouldn't work and would an unfun bag of hit points) and now somewhat loudly complains that another PC seems overpowered (even though they're not unkillable and have a rep for dropping every fight).