If you buy a cellphone and contract for service for that cellphone with a licensed carrier, then that carrier's license covers the operation of the phone you activated with that carrier. You can't then go out and use other phones under that contract without the carrier's consent. Likewise, you can't just build a new cellsite, operating on the carrier's frequencies, to provide coverage to your phone in an area (e.g., your entire city, neighborhood, backyard, basement, or office) where the carrier's network doesn't have a good signal. You have to have a license to build a base station or repeater, or an agreement with the licensee(s) whose signal will be transmitted that allows you to use their frequencies.
An "enhancer", "booster", or "repeater" is a transmitter (it may be configured as a broadband receiver and linear amplifier, but it's still a transmitter). If it isn't very low power and compliant with Part 15 limits, it requires a license, for good reason. Even well engineered, professionally installed transmitters can cause interference, either to other parts of the same network or to other networks. When interference occurs to the same network, the interference can be managed, because the engineers can tweak the power levels and frequencies of the various transmitters under their control to minimize the effects of the interference or cause users to operate on specific frequencies in given areas. An independently operated transmitter or "booster" of more than minimal signal strength in the same band that "repeats" the signal received at a given location without the carrier's knowledge or consent can wreak havoc on signal quality for other users without the network engineers being able to manage it. Yeah, Joe Blow gets a better signal in his back yard, but causes service to suck elsewhere in the area, and the carrier can't fix it unless they find out who's got the unlicensed transmitter and make the owner turn it off.
As I mentioned, even a well-installed transmitter under the carrier's control can cause interference to other networks, and obviously an independently installed transmitter may also cause interference of one sort or another out of band, especially since it won't be manufactured and installed to the same specs as a carrier's network equipment. There are any number of reasons for this, ranging from intermodulation products to desensitization of licensed transceivers. But if interference occurs to a police radio from what appears to be a cellular or SMR tranmsission, the FCC is going to look to the cellular or SMR operator to fix it -- but they can't fix problems caused by third parties using illegal cell boosters. They may have to turn down or turn off cellsites serving thousands of users, not knowing that the problem was caused by an illegal booster. Or the interference may be traced to a particular location by the FCC or the interfered-with party and the carrier is then requested to fix the interference, only to inform them that the carrier doesn't operate a cell at that location. This has actually happened.
Michael D. Sullivan Bethesda, MD (USA) (Replace "example.invalid" with "com" in my address.)