Because like, Return gets more powerful if the Pokemon loves the trainer a lot, so is it affected by the Belly Drum +6 Atk boost?
Yes, it is affected because Return is a physical move (whose base power is calculated by friendship), so Attack boost or drop can boost or drop the power of Return. A lot of move descriptions can cause slight misunderstandings so it's best for you to not overthink it much.
Source: Expericence