These repeaters are most definately radio transmitters. So they most certainly come under the approriate leglislation. The legislation has a very long history and is pretty severe for a lot of reasons. Essentially, in most countries, unless you have an approriate license you can't operate a radio transmitter. Certain frequency bands have been opened up, and certain certified devices may be operated by ordinary punters in those specific bands, 2.4 GHz is a well known example, but the devices that may be operated must adhere to specific design constraints. Even though 2.4 GHz is available without a license, if you try to fire up a non compliant device in that band you will be quite illegal.
A lot of this legislation is to stop idiots messing things up. And just buying a celluar repeater and installing it could easily involve an idiot messing things up. Whilst the idea seems obvious and easy, the devil is in the details. One of these repeaters could quite easily take a cell down, or at least degrade performance of the cell. For instance the cell handover protocol is rather dependant upon knowing that cells that are not adjacent are not going to try to hand a handset to one another. Willy nilly installation of repeaters could easily violate this assumption, leading to instability in the handover algorithms. Unlikely, especially just boosting signal inside an office space, but quite possible.
The idea that an operator could be involved in the private purchase and instalation is good. But by doing this the operator takes on legal responsibility for any problems that the repeater causes, including interference with other operator's services or any other interference. That is part of what getting a license for a transmitter is about. It isn't just a revenue raiser. It is the licensee warranting that they are competent enough not to mess things up, and taking on legal responsibility for the mess if they do.