Hamas is a designated terror organization. As such, it's illegal for them to have a Twitter account. It's also a breach of Twitter's own Terms of Service. See here for a comprehensive explanation: http://blogs.timesofisrael.com/kick-hamas-off-twitter/
Hamas is a designated Foreign Terrorist Organization (FTO) under s219 of the USA’s Immigration Act.
It is illegal for a US citizen or company to provide an FTO:
"…any property, tangible or intangible, or service, including currency (…) lodging, training, expert advice or assistance, safehouses, false documentation or identification, communications equipment, facilities, weapons…"
It is not a right under the constitution to provide terrorists free speech to promote terrorism.