I think there's a line between retaliating against someone, and refusing to help them in the future.
I do not believe that refusing to do business with an individual, where your business provides a non-life-critical service, is retaliation. A water company refusing to provide water to your home would be problematic. A luxury handbag store refusing to allow you to purchase more luxury handbags would not.
Image as a hypothetical that a customer goes into your store for the sole purpose of wasting your support staff's time. They are not going to make a purchase. They are also not directly committing a crime. They are just hurting your business for no particular reason.
Should you, as a business owner, be forced to allow them to continue to be on your property?
I think the ideal answer is yes for critical public spaces, and no for ordinary retail.
Steam clearly falls into the latter category and should be free to ban customers for any reason save discrimination against protected classes.
It's certainly retaliation if you can't use something you already paid for.
> I do not believe that refusing to do business with an individual, where your business provides a non-life-critical service, is retaliation.
This isn't accurate. It might not threaten your life or pose any great hurdle to overcome but retaliation has nothing to do with that. If they did it in response to an action you took not to solve a problem but instead out of spite or to otherwise get back at you then it is retaliation.
That isn't the same as refusing to do business with someone who isn't productive to associate with. The two are entirely separate categories.
Of course any business (including Steam) will attempt to argue that an instance of the former is actually the latter, and a difficult customer will attempt to argue that an instance of the latter is actually the former. Regardless, Steam (and most other businesses) behave in a clearly retaliatory manner regarding chargebacks. In cases where the company failing to respect the individual's legal rights is what led to the chargeback that shouldn't be permissible.
To frame it in the terms you used, any otherwise legal activity stemming directly from the company having violated an individual's legal rights should be treated in the same way that a protected class is.