They block it because a lot of managers and executives still think that blocking their employees from everything but their work is the best way to ensure worker productivity
You aren't 100% right. Some places also block any place where you can communicate/share/upload potentially confidential/proprietary/private/blah information. FB and Google+ certainly fall into this type of category (although, you could just pick up your cell phone, call a friend, and have them post whatever revealing information you have).
The issue is that in certain circles (e.g. in finance) all Internet communications must be recorded in some way, shape, or form. Google+ has yet to expose an API to do so, hence in a lot of places this cannot fly. Same deal with facebook chat and most other services you can think of. It's not entirely a productivity issue.
Corporate messaging systems (e.g. bloomberg) have special setups exactly for this issue.
Edit: check out SEC ruling 17a-4(b)(4) regarding recording for registered broker-dealers