An assumption about another person’s time can create an obligation they never accepted.

An obligation can be created entirely inside your own head. You assume somebody is free, build a plan around them, mention it to other people, and by the time they hear about it, refusing carries a cost they never agreed to. Nothing was asked, so nothing could be declined. The unfairness sits in the sequence rather than in the request itself.

Ask before you plan rather than after, and make the question genuinely answerable with a no. If you have already assumed, say so plainly and hand the choice back.