This is not a reaction comment as I've thought about the plugin for a while before writing.
I think this code is potentially intrusive and prying. Although I understand your rationale for the developer to track where his/her plugin is used, the concept of secretly reporting back site URLs is not IMHO the right way to do it.
some websites are deliberately covert, not for sinister reasons only, but as a matter of security and sensitivity of content. This code has the potential to expose anonymous websites without their knowing.
For example, I have developed sites for the Diplomatic service and my clients insist on minimal tracability except to members. OK, wordpress is not the best pl;atform for that, but this development makes thinks worse. I will personally check for this code and strip it out if found.