Every one of those addons is available directly from the vendor. The addons just set some ENV variables, or auto installs a plugin on deploy. Plus the provisioning apis are well documented so you could easily replicate it on a competing service so existing addon's would work with no additional dev effort needed by the addon provider (I've deployed a heroku addon).

Yep, good point on the free apps. I don't think a competitor would have to offer free apps though if they were able to lower the cost for paid installs and use the exact same deployment tools.