By representing itself both as an open platform and as a company with progressive values, Twitter has put itself into an awkward in-between spot and is bound to create such controversies.
Twitter and Facebook are starting to approach the threshold of 'public good' wherein at least, there would need to be rules or regulations.
If TW and FB did not actually regulate their content - we would see this exposed much more quickly. Foreign/Russian interference in elections would immediately force Congress to act, there's just too much power.
Aside from the ambiguities of 'how and what to police' we do have the added complexity of the nature of 'large, ostensibly public platforms' managed by private companies.