Twitter is selling a product, and they have a right to decide what kind of product it is. They have a right to decide what their brand is connected to. If you argue that free speech requires private companies to be neutral toward everyone who wants to use them as a medium, you're also arguing that newspapers must publish any editorials that are sent to them. You're arguing that TV networks accept all ads, no matter how repugnant, and that phone carriers allow stalkers to contact anyone they wish.
People should be willing to die to defend free speech, but let's also be clear on what is and isn't actually an attack on free speech. Yiannopoulos is a troll, and Twitter (in the interest of their shareholders) should have started banning people like him a long time ago.