Apparently it's just one part of a bigger plan, that includes adding paid tiers: http://mailinator.blogspot.pt/2013/05/mailinator-is-for-huma...

Personally, I just have to thank the creator for a great service that I used all the time (until I got my own domain). And for his technical blog posts: http://mailinator.blogspot.pt/2007/01/architecture-of-mailin...