Here is the official policy from Viejas Arena:

Any seat cushions without a rigid (e.g. metal or plastic) post will be permitted into Viejas Arena. Any guests with cushions or back supports with a rigid post will be asked to return it to their car or keep it in property check-in.

Viejas Arena seats
While most seats at Viejas Arena are chairs with a back, the highest rows are bench seats