The Taliban is somewhat of a different situation - they are the de facto government of Afghanistan, and are far less extreme than ISIS. While they have supported anti-Western terrorist attacks in the past, they claim to have changed, and it looks like their claim may be true.
I still wonder about the legalities of Twitter allowing them to use the site, given they are still under US sanctions. It is possible, however, that the US government has (quietly) asked Twitter to allow it, as a diplomatic/political calculation. Sanctions concerns disappear if the government is asking you to disregard them (they can give you a formal legal exemption from them - even secretly; even without a formal exemption, if the government asks you to do something, that is an estoppel against them taking legal action for acceding to your request.)
I still wonder about the legalities of Twitter allowing them to use the site, given they are still under US sanctions. It is possible, however, that the US government has (quietly) asked Twitter to allow it, as a diplomatic/political calculation. Sanctions concerns disappear if the government is asking you to disregard them (they can give you a formal legal exemption from them - even secretly; even without a formal exemption, if the government asks you to do something, that is an estoppel against them taking legal action for acceding to your request.)