Few things.
One: Teespring isn't a speech platform in the vein of twitter or youtube.
Two: White supremacist shirts have been banned for a while now (to my knowledge. If they haven't, link me the product & I'll resend this statement)
Three: We're laughing because the exact same argument you (Antifa/far left) have used to remove people's income and ability to speak freely online has been turned on you and you're absolutely livid. Even if we disagree with Teespring's actions, we're not going to pass up the opportunity to laugh like hell.

The trouble arises when large private media platforms become powerful enough to control the bulk of public dialogue. A powerful body of authority eliminating speech and press they find inconvenient is the same regardless of whether it's government proper or a semi-monopolistic corporation.

Tee-Spring is not one of those. Merchants should be free to refuse to carry products they find compromising.