Yeah it's actually sometimes an issue for us (the SO and I) to find providers since we have to try and determine if a lady is actually bisexual or just lists it to try and get more business. Thankfully P411 has a good search engine! It is sometimes interesting to see the ladies who are listed as bisexual and not couples-friendly (and vice versa).
So, given my experience so far, I would say no, most providers are not bisexual.