Cellphone signal boosters are regulated by the FCC like any amplifier or device that uses any frequency for amplification purposes. This means that there are specific laws to what is possible with any cellular amplifier ranging from what antennas can be used, how much gain can be created by the amplifier to even registering the amplifier with a provider. All these laws are to help the FCC regulate and protect from other cellular towers, providers, and even emergency responders to communicate effectively. If not followed, they can affect cellular towers and other frequencies that may be detrimental to communication for many users. These regulations were revised in 2014 and may have more revisions in the future.
Here’s a link to understand more about these FCC guidelines and restrictions for use and practice in all applications: