The reason given by the person in question was politeness. I'm not talking about whether or it's polite to change this setting.
> For instance if it is considered that an adult who's not autonomous can express consent and doesn't want to take their pills, social workers normally shouldn't (and legally can't) force them to take them, even if it's good for them.
That's because 'I want to die' is a valid reason. Or at least it should be. I doubt that social workers actually come to their conclusions from that framework. They probably do it like you said; simply acting against autonomy is itself bad within their framework, but this is inconsistent with society's behaviour at large.
Similarly, if GP's grandmother's reason was 'I want to receive random spam calls and calls from telemarketers' or 'I have friends overseas who might not be able to reach me', then I'd have nothing to say. Those are valid reasons. On the other hand, perceived potential impoliteness, which actually isn't, is not.