Having studied presidents for a while now and having worked for Congress as well I'd say there is a flaw in your assumption; that "they" is vague.

"They" is staff and from my reading of multiple presidential biographies staff is both protocol and manager of a politicians time and exposure. It can refer to everyone from Chief's of Staff to the Secret Service. "They" might not let you go and campaign for a friend in his/her district (Roosevelt used this a lot). "They" might not let you walk to the White House (Truman). "They" might not let you eat steak after a heart attack (LBJ).

"They" are always the people that tell you that time's up in a meeting. While this might be someone anecdotal from personal experience I think it is something they've taught incoming freshman in Congress so it might be ok to share. I know that the protocol for extracting a member from a meeting is to wait for a Member of Congress to be speaking. The staff member interrupts the Member not the guest about 3-5 minutes before time is up. "Congressman we're running up on our schedule", I say. The Member then says, "Just give me a few more minutes" somewhat hostile to the suggestion he should be going. I then stand up and move for the door. The Member then says, I guess "they're" saying we have to wrap things up.

They=Staff and staff manages the elected always. But the rules are prearranged with the elected so that there is a complete set of rules around all possible exchanges. In the Pres. Obama's case he was probably told that having lost both house's he needed to do what was originally coined during the Reagan Administration as "going over Congress' head" to the people. So while Reagan did it with speeches, contemporary presidents can do this with the internet, in this case Twitter. Most likely his staff in consultation with him said, you need to go over Congress' head and Twitter is one channel with which to do that. Which is what has brought us to "they're finally giving me my own account." I hope this answers your question.

edit: I need to go through this later and edit extensively, I was half asleep typing this on my phone before getting ready for work.