I'll respond to your question with a question: Why don't you have to pay to send e-mail to GMail users from a non-GMail host?

Because email is a decentralized network! iMessage isn't! iMessage isn't email! It's nothing like email! Also if an email provider wanted to charge other email providers to federate with them, they can. No one would pay it because there's too much competition for email services - it's hard to compete with free - but nothing is stopping anyone from charging for e-mail peering the same way ISPs do for regular internet currently.

Because google is an advertising company, and makes money by spying on users.

Google being found to scan emails received in gmail and advertise on that basis is why amazon receipt/invoice emails no longer include details inline.

GMail is not a free product. You are paying google with incredible amounts of your "private" information.

Because Google chose to implement an open protocol and to operate their email service as part of the broader email network. Apple chose a different path with their iMessage system, and that's fine too.

Why Gmail users don't have to pay to send e-mail to your mail host?

Wouldn't even have to be free. But currently there is no option.

It would have to be free, otherwise they become customers. The current option is to pay for an apple device.

Ok, should Sony be _required_ to support playing playstation games on an Xbox if people are willing to pay?

Communication protocols and interactive 3D software applications are not at all equivalent.

Going down the protocol route here - this would be more like Sony blocking game developers from allowing interoperability between Playstations and Xboxes. Crossplay is most definitely a thing - without requiring Xbox versions of games running on Sony hardware.