If we use the phrase based on MAC addresses, although it might have the correct meaning for the bridge as a whole, it conveys the incorrect meaning regarding the mechanism used for individual frame delivery. The pedantically correct sentence (which has been elided) would be:
Bridges operate at the data link layer and normally deliver frames within the same broadcast domain based on the MAC address contained within each frame.
This makes it clear it is a singular MAC address that is used to make the delivery. If we changed to the plural it would imply that several MAC addresses are used in the delivery of each frame, which is incorrect. Yes, many MAC addresses are used in the delivery of many frames but the choice of singular rather than plural conveys technical information.