Haha that's part of it. Some of the big players like mailinator, 10minutemail, Guerrillamail etc... seem like they have so many people using it they don't mind if companies flag/block their temporary emails. There's a lot of other devs out there that make sites like this (not naming names) but it's really just them practicing their coding skills. They're not putting a lot of time and effort into making a sustaining business service for people to use in the long run.
We're so tired of people thinking that they have to give their personal email address every time a company asks for it, so we're spending a lot of time and resources making a service where the email addresses work for any site.