This technology has been around for ages. We took part in a trial some time ago for Internet access and for networking a small office. It has loads of problems attached to it, not least mains spikes, general interference and noise. It caused so much data corruption and loss we gave up with it after one day.
No doubt things will improve but not for a while.
To answer your point, though. You are given a frequency to operate on and it is assured that no-one nearby has the same frequency. The interference and cable losses of such a small signal as is generated in a system like this means that the data is dissipated or swamped by the huge power capacity after a fairly short distance so there shouldn't be a problem of eavesdropping.