You have to look at it from the Trump teams point of view. They honestly don't see that the 'free speech' argument holds water. They see it as abuse of a public position to spread falsehoods.
Basically EPA employees in the Trump Team's view are perpetuating climate alarmism ( again in the TT view ).
So they are stopping that.
Just flip it around just for a second. I'll choose an equivalent that would cause liberals to shut down the twitter of an official.
Imagine a Department of Health official was tweeting photos of aborted foetuses and keeping a tally "350 aborted this month. #whatawaste".
There would be calls to stop that official speaking out.