It's your server. You're free to do whatever you want. You can serve different versions of the page depending on the UserAgent (has been done many times before)

It's your server. You're free to do whatever you want. You can serve different versions of the page depending on the UserAgent (has been done many times before). You can put up a paywall depending on UserAgent or OS (has been done). In short, it's a 2-way street: the client on the other end of the TCP pipe makes a request, and your server fulfills the request as it sees fit.