It's not a tax, but simply a charge to be on the platform, just like any consignment style shop gets a cut for the sale. It costs money to run the site, and tehy need some way to do it for free. The numbers may seem large, but someone paying a million in a year to substack is taking home 9 million. 10% is not a bad deal; but calling it a tax makes is somehow seem evil. If the writers can find a better deal elsewhere that generates the same revenue for less, more power to them; that is the beauty of competition.
As for the "storage and network access cosst are low" argument, a product is priced on value, not cost. Generating large readerships that make a lot of money for an other has value beyond the actual costs to the company that does that; and if you buy the costs determine price argument than charging $10 for a newsletter that costs nearly nothing for the nth copy means the writer should also charge a lot less as well. After all, why shoul they make a million dolalrs for something that cost them mayber a few hundred thousand (assuming 2000 work hours at say 100/hour) to produce.