Since TF2 for Linux was released there has been a growing underclass of intelligent idlers who were beginning to do it at a seriously massive scale. It's possible to run TF2 so it only uses around 150MB of RAM, which can be reduced even further by forcing the RAM to page out to virtual memory.
On a fairly standard £30 p/m server with 8GB RAM you would be able to run around 60 instances of TF2 and still have some fairly comfortable headroom in terms of server resources. It is also possible to automate the trading of items between accounts using the Steam Web Trading platform. Once one account hit the drop limit, they'd switch it out with another automatically.
This move affects those people. These people had 400+ accounts hitting the drop limit on a weekly basis, now they simply can't do that without triggering VAC, and turning off drops while in textmode induces the need for dedicated video hardware, and so on.
Valve has likely had this on the cards for a long while, but never implemented it because idlers were never a huge issue until it could be done on such a massive scale.
EDIT:
This guy had nearly 7000 accounts:
http://forums.sourceop.com/threads/145300-PSA-Idling-now-a-bannable-offense