I think it's a bit cliché; where do you draw the line? Should free software also display a copy of their license at first start and ask their users to click "I agree"?
When you start using a piece of software (free or not), there is a set of terms and conditions that you agree to (explicitly as is often the case with proprietary software or implicitly as with free software), which may include opt-out telemetry. As long as this is communicated, I don't see any problem with it.
To give credit where its due, I agree that Manjaro users may have never accepted opt-out telemetry when they first started using the OS and now this is being rolled out after the fact. Still, for a general-purpose OS that makes no privacy claims (e.g. Tails), I don't see how collecting their screen resolution etc makes a big difference. An average webpage today collects more than that in a single page view.