Either way, you're right about needing to open it back up. The impact to morale overall must have been huge when these people had to scramble to protect their livelihood.
* [1] http://www.businessinsider.com/ex-yahoos-confess-marissa-may...
Well... if you have, say, 80 people working for you, and you discover 15 are dead weight... cutting them would mean you can do your division's job with 65 people. You wouldn't have any room to argue for budget increase. That's a cynical view, but one I've seen played out a couple of times over the years in my travels.
VPN usage statistics. We have to take Y!'s word for it, but most news stories about the policy change say that Mayer looked at usage statistics to find that many work-from-home employees were working less than full time. I can't find a cite right now, but I thought I read that several employees would go weeks without logging in once.
Shitty Yahoo products.