1. You can run Linux applications on elementary. There's nothing forcing anyone to use the store.
2. The split is actually higher than you said for cheaper app apps because the minimum cut for elementary is $0.50.
3. There is a review process which I assume involves human effort.
4. Significantly, there is no enrolment fee and no subscription cost. So you could consider this a referral / hosting fee for elementary.
This seems reasonable actually. Projects need money to run.
I may be wrong about this, but at a quick glance it looked like stuff that would be mostly included in other desktop environments.
This is just my impression from skimming through it.
I wouldn't compare elementary OS's fee to that of Apple and Google.
Apple and Google take that 30% and keep it as profit, while elementary put it back (some percentage of it, I assume) into the project in the form of bounties a BountySource to fix bugs and improve the project. The more the project improves, the more users it will have, the more your app will sell, etc. etc.
Also, your bitter downplaying of the project by calling it an "experiment" vs. Apple and Google and the whole negative tone of your comment is pretty annoying. You're the one comparing the project to the biggest tech companies in the world, not them. Weird.