I don't want to get into a long dissertation on search and seizure law, which is not only hideously complex but constantly changing.
However, items in plain view are not subject to the same "privacy" concerns as items say, in your house.
Likewise if the homeowner was standing in the garage, it's certainly appropriate for officers to approach this person to talk to him.
You have no more expectation of privacy in that case than you would from, say, the mailman.
However, you don't have to talk to the officers. One can only surmise that the officers might have gotten a call from a neighbor or passerby to the effect that "that guy has a lot of bikes in his garage, he must be stealing them".
The above poster is quite right, most police officers are not familiar with people having a lot of bikes as we enthusiasts are prone to.