Yes. You're the one who chose to use their platform to broadcast your words, if you don't believe they will do so with fidelity use a different platform. You're not paying or otherwise offering them anything in exchange for a guarantee of fidelity, nor are you under any obligation to use their service.
So by your logic, it would be okay for a twitter employee to modify a tweet from of the president's account to declare war (or to any number of things that would have very real repercussions in the real world)
I take no issue with the modification. Trying to start a war is wrong, but that's true regardless of method. Giving someone food isn't a problem, giving someone food you know they are deathly allergic to on the other hand is. They would certainly be responsible for any damages they caused through malice or negligence, but they have the right to face those consequences.
Why not, if they had it in their terms of service?