Corporations, Schools, and a host of Non Profit organizations block internet sites of all kinds .. At my biggest client you cannot go to an ebay or facebook or twitter and certainly don't try smut surfing ... why ... cause they own the machine and it is for "work" not play ... many also block email such as gmail or yahoo mail.. again it is within their rights and completely legal .. If you want to do that stuff ... do it at home not at work ...
In addition all the big corps that I work with maintain history records of every website their employee's visited .. they do that for legal reasons and for work reason ... right or wrong they also can read any email that comes into their machine ... again that is legal ..