Well that's as vague as an incomplete strand of the Draconic Prophecy.
Is this gonna be one of those cases where one person is running with a pants-on-head interpretation of RAW in questionable faith, and the second person is trying to explain RAI and designer intent to them but is getting the piss taken out of them? Is this a "Card says Moops"? Only with setting lore this time?
Because I feel like I'm the second person right now.
And people wonder why diehard Eberron fans place Word of Keith above Word of God. Usually because Word of God (WotC) isn't vetted all that well.
I don't think this is nearly as bad as you are making it sound. I think it is just a great deal of nuance here.
For example, in the text
@Paul Farquhar quotes, it mentions the House Ghallanda having it's mark in every inn to ensure "quality" .
So, on one hand, it is explicit in the setting that if you set up an inn to compete with House Ghallanda, they aren't likely to do something like kill you out of hand or arrest you for simply opening a business. They aren't to that level.
But, on the other hand, that inn will likely fail and leave the owner bankrupt, because everyone in Sharn knows that Inns without the House Ghallanda mark are inferior, they do not have the same services, they charge more, you can't have the stablehand transfer your horse from one inn to a different one for free because you bought the rewards program, ect. You can't be just good to run a business without the House's approval. You have to be the best, better than people literally boosted by magic.
But then, we get into some interpretation. Let us say you are boosted by magic (by having the mark) and you are successfully cutting into profits. What do they do? Well, it depends a little on the personality you give the people in House Ghallanda in your version of Eberron. If they are more like the Robber Barons and corporations from 1920's America, they likely will get the Watch on you. Perhaps the Watch just constantly gets tips that there are smugglers staying at your establishment, because of course an unregistered inn would attract seedy folk. Perhaps vandals break in and break all your tables, you just don't have the same security after all.
It is a legitimate view of the text, they are a monopoly, labor laws are not in existance, and they are cut throat in their competition. It is reasonable they use some of the same tactics we know businesses have used for centuries to take out their rivals.
But, your view is also right, if the business actually isn't cutting into profits, if the cost of breaking them is more than the cost of just ignoring them, then they probably won't get involved beyond just market pressures that cost them nothing. Because you are too small to be worth their effort.
It depends on how you approach the scenario.