When I lived in Boston I played a lot of pick-up basketball in the local parks. Each court had its own particular set of rules. I sometimes chose the park I would go to because I wanted to play one way, as opposed to another. I think the same thing applies to jams and open mic nights. Each venue has its own rules, traditions or idiosyncrasies.

I have been to open mics where unsolicited accompaniment would be rude. But I've been to others where the 'house' musician or musicians were expected to accompany any and all acts, much like you've described. I ran an open mic for several months in Dallas, and were somewhere in between. We would offer each guest the option.

Sounds like your accompaniment was appeciated, and I suspect that the request for a capella was quite reasonable, and not a breach of etiquette.