I actually thought he rented those terminals out at $20,000 a month not annually. He basically has a monopoly and Trump should let the antitrust division loose on him.
Actually, $2,000-$2,500/mo. is what I recall - as in the article. There was also add-on, subscription content that could add to that - research, quotation services, market feeds, etc. It was the original content aggregator on Wall St., and focused exclusively on that community. You literally could not function in the trading world unless you had access to a Bloomberg terminal.
As others noted, all of that information passing through a centralized computing structure would provide someone with less-tha-pure motives the opportunity to capitalize on this non-public information.
Another major feature - and one that made the service invaluable - was the built in messaging service. This gave any user the opportunity to communicate directly and privately with any other user. Were "tips" shared across this network? Certainly! Wall St. has always been in insider's game, and all the players rely on these terminals to play the game.
He doesn't have a monopoly. Thompson-Reuters is the main competition and has their own slightly cheaper terminal for rent. In the financial information space, Bloomberg has about 33% market share to Thompson-Reuters 23%. Then there's smaller players like Moody's.