Seattle's system had peak hour restrictions for a while, I think it was something like no bike loading/unloading in a specific section of the downtown corridor. I always guessed that they didn't want to slow down boarding times, but I think it takes longer for people to board the bus during a busy period than to put a couple bikes on the rack. Could also have been that the buses only had so much space to stack at curbside stops.

I'm a bit fuzzy on the details since it didn't impact my commute (I never worked downtown), but I think after the bus tunnel re-opened (sometime in the last 2 years) they began to permit people to use the bike racks downtown at any time.