Firstly, not everyone wants to use or install a proprietary system.
Tapatalk annoys users by prompting with an alert (instead of the recommended way to do it, a smart banner). Forums with tapatalk becomes a walking billboard for their software, and while it does benefits people who use tapatalk, it presents a negative user experience for other users.
I wouldn't mind tapatalk if it stored "no thanks", but it asks you every session.
Bear in mind that any forum owner with a little bit of programming know-how could easily disable those notifications for users...
