It’s essentially just shareware.

More specifically, it’s nagware which wasn’t particularly uncommon for the time WinRAR was introduced so I don’t know that it’s particularly generous really when one considers all the other nagware that came out in the late 90s.

It’s just one of many different licensing strategies.

In this case it seems to have paid off for the developer as it appears to have resulted in a great deal of fondness and goodwill among a certain portion of the user base.