I am guessing your are referring to a hypothetical situation like this: Let's say you have a property and are required to pay $100 a year to have it registered, and $50 a year to have it inspected, and have to spend some other annual amounts that are required.
In addition to this, let's say you have a 15% hotel tax you are required to collect and remit from your guests (Air does not collect it).
So much of your competition has 15% lower rates because they do not collect the tax, and they also have extra money to pamper the guests because they aren't spending it on the annual required regulations.
If so...that's a tough call. I think I would first have to have first hand knowledge that those hosts are 100% aware of what they are supposed to be doing, before I would report them. A lot of hosts have no clue that they are supposed to be collecting hotel tax, so I would feel bad for getting them fined, etc. especially for back taxes owed. But it would be a different story if I overheard them bragging about getting away with it.
Now if they don't want to pay their IRS taxes, that's between them and the IRS, but if most of my competition wasn't collecting hotel tax on purpose...then yeah I'd be a bit pissed about it!