That's a bit intrusive, but users sign up for a free service they have to pay in some way. The Internet isn't free.
Caveat emptor is a deprecated concept in Europe. In general, the law assumes consumers are basically children, and need to be protected from themselves.
Increasingly, "small print" is not deemed sufficient to inform a consumer about rights or obligations arising from a contract. There was an intermediate "Key Features" stage where significant rights and obligations were highlighted, but even this is non enough to protect childlike consumers from dangerous, predatory companies.
Now, significant rights and obligations require explicit, itemised opt-ins. If you thought the cookies thing was a PITA, you wait for GPDR
Consumers are actively encouraged to believe the State will protect them from any consequences to their own free choices. Consumers get dumber as a result, meaning the State is required to intervene more often- causing consumers to become even dumber. A viscous circle in which more power accrues to the state, while personal responsibility bleeds away from the individual.
I get that WhatsApp users who registered before the company was acquired by Facebook might be peeved if data was moved into the FB ecosystem, particularly if they don't use facebook. But for users of both, or those who signed up after the acquisition, I'm not sure where the harm lies.
I'm a big advocate of Privacy, but it's naive in the extreme to expect a data company to not use the data you chose to give them.