> What do you care about ability-to-pay?

Because it helps align incentives. You can have a 7% tax on sales, or a 5% tax on sales plus a 2% tax based on a noisy correlate. The latter will raise more revenue at a lower excess cost.

> You can't shield from that tax. All entities pay, for every sale, no refunds.

That would make it a tax on revenue, which is quite distortionary indeed and generally considered a last-resort. (Or you could have a VAT, where all entities pay but tax paid on non-final sales is refunded. VATs are quite good but not totally free of this kind of gaming.)