I remember at an old job on a stock trader's last day he emailed himself (from corporate email to gmail) a spreadsheet that contained proprietary models, client holdings, etc. That's a serious breach, and luckily traders are dumb enough to use corporate email to do this because if he used something like dropbox it probably would never have been caught. I don't like being restricted ever, but you can see why a company might try to block these cloud storage services to protect itself and its clients.