I was stumped when presented with a UAC prompt trying to start the game. Running stuff as admin is a no go here (especially since that would block the kids from playing anything :-) and should not be required by any modern game, ever.

I removed the RUNASADMIN flag, and the game just runs fine (Win8/64). Now, what's the reason it's flagged to be run elevated?

For older games that used to store saved games in the games installation folder, admin privileges are necessary if you installed the game in "Program Files". This is because, since Vista, Windows is a lot stricter about what programs can do without explicit user permission. (This is a good reason to have your games installed somewhere else, not in "Program Files")

But newer games normally store saved games in the "Users\<YourAccount>\AppData\Local" and Magrunner probably does the same (I don't know). So I don't know why it asks for admin privileges, but this is something I've seen on all GOG games I installed.

It's a good thing you're being responsible about this, though. There's no real reason to be worried about games, but in general it's a good thing to not grant applications any and all privileges they ask for.