Everything is subject to sandboxing if it wants to sell through the Mac App Store. He could also just not do that, and have none of those limitations.

It's a restriction of the Mac App Store, not the actual Mac computer itself. Mac is just Unix with a pretty UI and lots of features.

Technically, that's true of course. I myself use the “computer” part only and shun the “AppStore” part as much as at all possible, but Apple is renowned for the deep integration of all kinds of components and there comes a point where you just don't have a choice any more. I seem to see the area of free choice getting smaller with every OS release.

There was no need for the developers to release this on the App Store. Anyone installing a Haskell IDE is perfectly capable of installing an app normally.

They are charging money for it, so that is reason enough to use the Mac app store. If it was free, then I totally agree.

They can charge for it on their own web site. No requirement to go through MAS to sell Mac software. If people are having a problem with this, complain to the author of the software.

Charging money is hard if you aren't a big company, which is why app stores have been super successful.