AFAIK, being disabled means you get to have a toilet that's usable to you, not that you get to avoid waiting in lines. If there's someone in line behind me who is obviously disabled and looks like waiting would be a burden, I let her ahead. If not, I take the next available one. I see nothing wrong with this?
Rebel has it right. Handicapped toilets are for the accommodation of handicapped people. There is nothing that prevents anyone else from using the facilities if they are vacant. Handicapped parking is another matter.