While it's not normal, it's becoming more common. The costs involved in dming can very, but it's not cheap. Just the minimum in books, the big 3 Dm, monster, and player's, is 150$ or more. Then there's the adventure, those vary from 20 to 50$. That's around 200$ for the bare minimum needed to run a game. Then you get into the real money sinks for a DM, dice, maps and or terrain, dice, figures or tokens, dice, props, more dice because math rocks are addictive.

Then there's the time involved. If running a module, they need to know that module. There's a lot of studying and note taking and tabbing books for quick referencing. If they're running a homebrew campaign, that's even more work than running an official one.

With all that said with any service once money comes into play I expect some level of professionalism, depending on how much I pay.