PADI Sidemount course (Rec and Tec variants) only became available as standardized qualifications this year. For some reason, PADI have spent an under-whelming amount of effort on promoting them.
I don't see the problem with expecting a qualification from sidemount divers. It's not a fair comparison to hold sidemount against equipment use like an underwater camera - sidemount diving does require a specific skill set.
That said, any dive operator with a semblance of dive community knowledge will be aware that sidemount is a relatively 'home-grown' development and there are many users who didn't receive formal training. In that respect, assessing the logbook is equally valid to the c-card.
I don't see why a dive operator would 'prevent' you from choosing to dive sidemount, should you arrive for diving with your own gear; only requiring tank rental.
I can also see why a dive operator would refuse to rent you sidemount kit, without evidence of training.
Will you be taking your own sidemount kit on the holiday? Or expecting to rent a rig?