It isn't a constitutional free speech issue, but it is still an issue of being able to speak freely.
What? Twitter is not preventing you from "speaking freely". They just don't want you to do it in their private business. You can go set up your own social network on your own server and "speak freely" all the hate you want.
Twitter is under no obligation, legally or morally, to provide a soap box for bigots and extremists. Just as a grocery store is within its rights, legally and morally, to kick out a person who is running up and down the aisles ranting racist shit. And a newspaper is under no obligation, legally or morally, to print a paranoid call to violence in its Letters to the Editor.
Everyone loves to pick on the SJWs (ok, admittedly they sometimes make it too easy), but there are a helluva lot of people on the other side with just as much of a Special Snowflake Syndrome, who think that everyone owes them a platform (for free!) to spew whatever hateful shit they want.