No, I don't think it's okay to be disrespected as a valued customer. Professionalism would require that staff and employees understand that paying customers is what makes their pay checks.
Of course, there's a limit to what they can handle, so if they think that a customer is rude, they have a right to firmly but respectfully state to not talk to them in a certain way, but still keep professional by striving to make it right to the customer.
On the other hand, I know so many people that don't know how to treat other people... It's nuts.
I work as an aid, and while not everyone can like me, I can assure you that I have a 99% success rate with clients. It's because I treat them as people, and as a guest coming into their homes to take care of them, well, I try to remember that these are people with families. If this person were MY parent, aunt or uncle, then how would I want their aid to treat them? That's how I do it. So, of course it's not perfect. Sure, I may make small mistakes, but, I correct what I do, and if I'm in a bind, I'll still get to air my grievances, if the situation comes up, and because I do have good customer service, then I'm taken seriously about a gripe.