The Secret Service works for the President. They are in no position to "allow". They may "convince", but that's about it.
Now I am worried...
If that is true then I am more than worried.
The Secret Service is a Federal law enforcement agency, under the Department of Homeland Security.
The Department of Homeland Security is headed up by the Secretary of Homeland Security.
The Secretary of Homeland Security is a cabinet position. The Secretary reports to, and serves at the pleasure of, the President of The United States.
The President of the United States is the head of the Executive Branch of our government.
And come Jan 20th, the President will be Donald Trump. Somehow.
If Trump wishes to continue to Tweet, the Secret Service is in no position to do anything other than ask him politely to use secure devices to do so.
You all will pull Donald's tweeting device only from his cold dead hands...