I'd say all the places I've worked have mail opened generally for a number of reasons, security, risk management (so complaints, problems etc... cannot be hidden or important issues missed and similar), logistics... with a few exceptions such as mail which looks like it's private for the boss, bank statements etc... Who knows what people could get sent to work as well, I'd say it's a prudent precaution.
When opening a load of post I'd say it's very easy to open something you maybe wouldn't normally so it may well not have been on purpose.
I'd say it's a perk being able to have mail delivered and fair enough having terms covering it (and ultimately the company), they could just as easily just say no personal post to a work address. I imagine it would be only too easy for them to revoke such a privilege should it cause issues they really have no need to have to deal with.
As others have said, if people don't want to risk things being opened by colleagues then they should rethink getting it delivered to a work address.