Or it's to protect nearby receivers. A Tesla Coil I built in middle school wiped out TV and radio reception for most of the block...

Just like a whisper in your ear would overcome someone shouting a few hundred yards away. It's not the radiant power that's the problem but the proximity to the origin of the transmission. And Tesla coils are particularly dirty in their emissions.

You could potentially switch to a mesh or some kind of resonant structure with large enough gaps that cell signals could get through while blocking the rooms designed resonant frequency.

That and the lack of windows might not be to everyone's taste.

FTA:

> Finally, the high Q-factor and sub-wavelength operation of the QSCR room permits the inclusion of windows and doors, without significantly altering system performance.

But there may be limits on how much area can be covered by windows...

Thin enough silver layer on glass is transparent.

Just coat them with ITO!

Conductive windows exist.