Amongst other things, it makes NSA tapping of fiber in front of their datacenters less useful. It also makes switches that replicate traffic to NSA less useful.
http is like the postcard of the internet. By switching to https only, they are requiring you to send them messages in security envelopes rather than postcards.
The http/https question is completely irrelevant to the PRISM-complicity question. I'm not sure why you brought it up.