It seems to me that framing the issue of a (possible) social custom in terms of whether there should be a rule that covers all situations is a debate tactic designed to undermine support for a custom similar to the all-encompassing one used in framing.

The answer to whether there should be a custom that always applies is pretty much always going to be no, which doesn't tell us about similar customs (like one of usually or often explaining downvotes) even though it seems like it does.