And it's their software, not yours. They are free to require telemetry if they choose to do so. You're also free not to use their software if you don't like it.
FWIW, I think it should be opt-in by default, but I think it's reasonable (aside from adhering to necessary privacy laws) for a project to choose the policy they want.
Arguably, the point of open source software (or at least GPL licensed software) is that the owner of the hardware gets to "own" the software and can do what they like with it.
In that scenario they certainly can. They just have to take the source, modify it to suit their needs, and recompile it. They are 100% within their rights to do that.
FWIW, I think it should be opt-in by default, but I think it's reasonable (aside from adhering to necessary privacy laws) for a project to choose the policy they want.