My guess would be that IT would block streaming to reduce load and/or internet connection cost. Especially if you have a lot of people in the building.
A quick google search says that Spotify is using 144 MB/h for high-quality streaming[1] (premium only), so that's roughly 1 GB per person per working day. That's quite a bit - without Spotify, I burn through roughly 500 MB per day when working.
