Everyone is allowed on Twitter, unless they violated the guidelines. I dont follow the Taliban so I have no idea if anything they have said violated guidelines or not. For all I know they are tweeting about What the best bread and cheese pairing is during a jihad.
You do know who the Taliban is though right? They are an extremist group that glorifies violence, rape, and zero women's rights as well as extreme racism towards anyone who doesn't share their religious beliefs. Did you happen to see why so many people were desperate to get out of Afghanistan when they took over?
But hey, if you don't know who they are then it doesn't matter if they are on twitter right?