Sorry, I have to quibble here. While group owners technically have the final say, some groups do function as a democracy, with 'owners' who merely contribute administrative duties to the group.
I've not had this particular problem with digests, but I agree that users should be able to choose whether they want plain text or html, and not have that left to group managers. My own problem with digests remains that context gets eliminated in discussion threads, which also affects our ability to forward mailings. I was initially really excited about Groups.io, and for the most part I still like it, but I wish users had more control over digests.
Isis