Why wouldn't you though, that's gotta be pretty juicy data. You can compare phone numbers across so many different databases now, makes profile creation 10x more efficient. Not really surprising that everyone wants your phone number badly these days.
Microsoft does the same thing btw. Was really fun for a friend of mine who registered a Microsoft account for mixer, forgot about, bought Halo, needed an MS account to log in, thought hey I already have one, and instantly got locked out because it didn't have a phone number.