I think there would be some reasons behind it, and I think the most reason is because they are arraid that people are using "alexamaster.com" domain to make alexamaster for other usage. It used to be nothing, which has no one was using it. Also, they can get double SEO's from google, because they have two domain, but that's not the most reason, because you can just create the subdomain by that. Also, Alexamaster.com redirect to Alexamaster.net while you clicked something, that shows that thir main site is Alexamaster.net not alexamaster.com. There might have other reason, but I think this is the reason.