They're not 'disabled toilets' they're 'accessible toilets'. That stick wheelchair symbol doesn't mean it's for disabled people, it simply conveys this information:
With two exceptions the designated area will generally, but perhaps not wholly, be accessible someone in a wheelchair. The two exceptions are toilets and lifts (elevators). In those two cases the entire area designated by the symbol, and all the controls, equipment and appliances inside, are accessible to someone in a wheelchair.
—ISO 7001, but paraphrased because I don't have the precise wording to hand, but it's accurate.
It says nothing at all about who is or isn't entitled to use the facilities, it simply conveys information.
It's only a convention that if someone isn't disabled then they should use the 'normal' facilities in case someone who cannot use those, needs to use the toilet. That's all it is, a convention, although it's polite and considerate, it's not a rule still less is it a law. If someone with, for example, a child in a buggy, needs to use the toilet and can't get into a normal cubicle because of the child in the buggy, then that person is fully entitled to use the accessible toilet and no-one is entitled to complain. Anyone, disabled or not, is entitled to use an accessible toilet.
Your local council's disability policy officer will confirm this, you don't have to believe me. Or you can buy a copy of ISO 7001 for £££, and read about international road signs!