"If local government can't use local taxes to provide shared toilet facilities to stop us all having to defecate in the streets then what are they (and taxes) for?"
Well, there are many different ways to tax things. This is a pretty big area of study in e.g. economics and public polciy.
Certainly, one way to tax is to e.g. collect income taxes from everyone and then use it to maintain public toilets. But another way to tax things is to directly tax the thing itself - like in this case, taxing the use of the toilet.
This has some drawbacks and some benefits. One of the benefits is that you're making the toilet more akin to a regular, private good - people pay for what they use, which works pretty well in most cases.
Of course you can appeal to emotion and say "what, we'll make poor people not able to use restrooms"? Clever as that is, what about the alternative appeal - are you really comfortable with a poor person's taxes going to support the public toilets of the 1%? The areas they live have public restrooms too, and I'm sure they cost a hell of a lot more to maintain than most public restrooms.
I have no idea how this actually works in the real world - do rich 1%'ers public toilets cost a lot? Are they supported by taxes (at least, not on the municipal level)? Is there any amount of poor people unable to find access to restrooms that this is even a problem?
I don't know. I'm just saying, the view expressed by Stallman is not nuanced, doesn't provide any data to back him up, only goes for a totally surface-level idea of the morality, and supports it with emotion.
NOT everything in the world is a totally clear black & white moral issue, and not everyone that disagrees with you is necessarily morally wrong! When you make every issue a moral & emotional one, you lose out on actual thoughtful intelligence, or on the idea that smart people can think of different ways of cooperating that might make sense to them.