IANAL, but if you make the product available in the EU, you are required to make telemetry opt-in if there is personally identifiable information, or you'll violate the GDPR. Note that according to interpretations of various EU bodies, PII also means information that could be correlated to persons, like randomly generated identifiers (not just obviously PII information like an e-mail address or IP address).
> Besides that, they probably have bigger fish to fry than some program that is used by a tiny subset of the general population.
That's exactly the reason why MS would be on the safe side and not break any laws in this case. They could be a bit more daring if it benefitted them, but in this case there's literally no benefit except improving the software that they are giving out for free.
Why? This isn't a product marketed to kids using dark patterns or something. People who feel like you do can easily opt out.
>If it is such a good thing, why developers prefer not to disclose its usage?
Maybe because they feel that it's not that big a deal. They could also "disclose" that the app uses X amount of harddisk space or memory, or that it was built by developers who are running Windows as their primary OS. That might be a deal breaker to some, but most people probably don't care.