Hey Nico,
Of course outbound mail delivery is a selling point of Postmatic Premium. We are a business and need to be sure we make enough money to continue developing and maintaining not just Postmatic but also our 3 other free plugins.
With all due respect I'm not obligated to give you any further explanations of our business model or technical decisions. We looked at other post/comment subscription plugins and what kind of support tickets they deal with most. Many are plagued by tickets asking for help with wp_mail, cron, or smtp plugin errors. We made the decision to bypass all of that and also gained other opportunities by having deeper level access to mail sending... which opens doors for future features that are not possible with wp_mail. It's been a fine decision for overwhelming majority of our users.
And, the obvious is that Postmatic is GPL. We created it. It's yours. Tweak it. You're even welcome to use our servers for your incoming comments.
Respectfully,
Jason