They are making that decision. Their decision is to act as a service provider that does not pass judgement on any users of their platform operating within the law. There is nothing wrong with this decision - it is a non-decision.
I'm more perturbed by providers that choose to take a side. Those providers are now implicitly morally approving of all remaining users of their platform.
That said -- there are products that Shopify does (and would) kick a user for selling. Shopify isn't neutral, in fact, they're saying Breitbart is acceptable.
No. Things that get banned almost always have significant political content.
Homosexual content (not necc. porn) comes to mind as a perfect example of "whole classes of goods and services" that are also very much political speech. "obscene speech" (see The Howl trial) is another famous example of an attempt to re-brand "political speech" as an apolitical "whole class of goods and services".
Taking some class of controversial goods and services and branding it as "not political" is just begging the question. It's saying "this thing is so obviously bad that discussion of its acceptability is beyond the scope of our current politics." IMO saying something is "beyond politics" is the ultimate political judgement.
If you look at list of banned goods and services, most of them are not politically related, but are related to liability, abuse potential and (probably) payment processor restrictions.
I'm not saying that banning whole classes of goods doesn't have an impact on free speech.
Yet there is still a fundamental difference between having a list of prohibited goods / services and removing a member selling allowed goods and services because you don't like their politics.
The first one is banning the sale of pornographic products on your platform, the second is banning the sale of (otherwise permissible) t-shirts by pornography companies on your platform.
They would have been better off not making a statement than making the one they did.
You claim that this argument is disingenuous, ie that the argument is not sincere. It seems sincere to me.
I think it's a pretty convincing argument. Free speech doesn't mean a whole lot if you can't buy pen and paper, or engage some web company to host your website, or sell merchandise (so this post argues).