This is nothing like sticking a big coil of wire underneath a big power line running over your property for "free" electricity (I assume that's what you meant by "telephone line power theft") In that case the power is supposed to stay on the wire, and one takes advantage of the fact that not all of it does and uses that wire to encourage more of it to leave the wire than would have otherwise been the case if that wire was not there. Thus the generator needs to increase its power to cover what you're stealing.
In the case of cellular, wifi or bluetooth signals, the power is not supposed to stay on the wire (i.e. antenna) but to be radiated into the air. Once it is in the air it makes no difference to the transmitter whether it is received and used as intended, received and used for some miniscule scrap of power, hits something resulting in a tiny increase in that something's temperature, or radiates into outer space. The transmitter power stays the same regardless of which of those four fates awaits an individual photon emitted by the antenna.