That's definitely a concern, but I feel this can be mitigated by running your own network on top of theirs. Anyone in my home is part of my network, doesn't mean they're in the wg network too.

Aside from that, it's definitely a problem that they could include themselves in any customer network, but the accountability still stands. If someone got in without your screw-up, at least you know who to point the finger at once the dust settles.

I'd argue it should be treated as a base to overlay your network on top of. Although admittedly I say that as someone that doesn't use their services for similar reasons.