Line-of-sight communications are definitely less susceptible to surveillance than WiFi that has omnidirectional broadcast. Even if the signal bleeds due to refraction, at such high frequencies, this loss of power is extremely detrimental to making sense of the waveforms.

Multipath interference can be managed with existing communications techniques, as light isn't the only EM wave that bounces off of things.

You can use a lens to focus the light to travel in a narrow beam, so each beam is separate and don't interfere.

A lens for radio frequencies is not practical (it's a satellite dish basically), so they use antennas that have a much broader beam.

My uneducated guess is just like wifi, you can use different frequency to distinguish between different "channels" maybe