Two things - you're not paying for apps in any way, they're FLOSS. You're paying for services - infrastructure, administration, "cloud" space etc.
Also, while Purism is indeed a for-profit company, it's actually an interesting case since it's a Social Purpose Corporation - it's not there to maximize profits, but to maximize its social purpose. It legally can't do anything that would be at odds with its legally defined social purpose: https://puri.sm/about/social-purpose/
I see your point, but as they’ve also created the apps under their brand it seems likely the monthly fee would also go towards (hopefully!) the continued improvement of those apps.
Judging from my past interactions with people from Purism I'm pretty sure their goal is to have all their improvements going upstream, with forks focusing mostly on branding and cross-integration within the bundle and their hardware.
Also, while Purism is indeed a for-profit company, it's actually an interesting case since it's a Social Purpose Corporation - it's not there to maximize profits, but to maximize its social purpose. It legally can't do anything that would be at odds with its legally defined social purpose: https://puri.sm/about/social-purpose/