Maybe you read Janx's post differently than I did, but I don't think that's what he's saying at all.
No, I think I get where he's coming from. It's the same from a character or player perspective.
You are saying the shopkeeper's players in the Merchants and Peddlers game should -assume- there are big bad monsters outside the city walls even though the DM is glossing that fact because it isn't what that particular game is about. Then perhaps the players in a DnD game should also assume their game world, full of towns, varying economies, shops of all manner have some sort of functioning economy with taxes, laws and such which the DM also glosses because the game is not entirely about that. And, presumably their characters don't involve themselves with it to a large extent.
If, in either case, the players want to play in the other "realm" (adventure or shopkeeping) they shouldn't hold it against the DM for adding more detail to said systems in order to make the game challenging. Nor should they be surprised if they find their characters are not suited for the challenges of the "other realm" if they did not plan for it. Perhaps Wizards, Rangers and such aren't best suited as entrepreneurs. Maybe they should have made Experts?
Is it a DM cop-out if the DM didn't sit everyone down and explain the taxation, business license and other laws at the start fo the campaign? I don't think so. Could a DM use this to ruthlessly squash players starting a business? I suppose so. But my earlier point was I don't think they would find it necessary with most adventuring parties. Most players would find sitting at a desk rolling "Quickbooks" skill checks pretty damn boring

They want to get out and hit stuff and take the treasure from some hard working Bad Guy...
To tie back to the OP - Long term making money with magic should be just like any other business. Usually if a PC can do it a lot of other people have done it before you (unless your spells are wholly unique to the campaign world). Therefore turning it into a buisness model to make money should mean competition, business plans, marketing, adhereing to or avoiding laws and all the other things all those people with "money maker spell x" have tried and done before you. If your players want that to be the focus of their game, great, more power to them, though I HOPE (DM included) everyone agreed to it ahead of time.
Like the attempt at using a Cantrip to turn pigs ears to Bugbear ears - its a cantrip so unless its a low magic world LOTS of people have access to the spell. Chances are the gaurds have dealt with something like this before and just because the DM didn't explicitly tell players "cantrip concealed ears aren't allowed" doesn't mean he is a jerk if the citywatch applies some sort of test to determine authenticity after the ears are turned in. Even if the DM hadn't previously considered it himself. I agree, doing this for every innovative thing the players come up with is just poor DMing. However, in some cases it is just common sense.