They aren't likely to get as many "donations" as they would placing books and mags in the door to door work, even thouhg most of the donation money comes from the rank and file themselves. Also the internet is not a hierarchial platform. If it was, and if access could be controlled top-down, as it can be on other computer networks, then no doubt the Watchtower would be offering an ISP service to its members on a donation basis. All they can do is have their few official websites like an outpost in a hostile frontier.
But they probably do use the internet more then we think. They spy on this forum, for instance. A few years ago they also had (but did not advertise) smtp.watchtower.org. So they were using a mail server. It's since gone silent. Had open telnet on it too. Big security hole, there.