Not to mention that if you are afraid of Google exploiting your data, you'd probably have the same problem with the NSA so cisco, juniper and almost all other US-based vendors (and why not Chinese/Russian?) are out of question.
Most corporations are more concerned with whether or not the data goes "off-network" or offsite at all, not whether or not the intermediary hardware they own is compromised by three-letter agencies.
At a previous job the building next door was a remote office for a non-IT part of a large non-tech company. They spent the money to get some kind of carrier line (maybe EPL/EVPL?) solely for teleconferencing.
While the system is almost certainly not difficult to compromise for TLAs and others, they get assurance that the data hasn't left "their" network,
I personally don't think there's much rhyme or reason to that particular permutation of those policies, but there is certainly a large group of buyers who are fine with potentially backdoored commercial products as long as "the data doesn't leave our network" and hence would not be OK with Google's solution.
Webex and GoToMeeting handle all-hands type meetings by making it only a 1-way presentation (view-only as you said). Participants can only ask questions via a chat dialog. The nice thing is, it prevent stupid feedback loops you always get when someone forgets to mute themselves.
Though, I believe both Webex and GoToMeeting charge extra to have this ability.