Originally Posted by Kevin P. Egan
I believe it has to do w/ the operating frequency of the phone. A lot of phones operate on either 900 mhz or 2.4 ghz frequency. The 2.4 ghz frequency is also used by wifi wireless devices, and a cordless phone could potentially interfere with signal. Also, if a lot of people in a small space all start using cordless phones which operate on the same frequency, there's going to be signal disruption.
But I am not sure if that's the official reason
That is the reason, and your other point applies as well. If a bunch of people came with cordless phones, all operating on the same frequency band in close proximity, they wouldn't be very useful anyway.