Do you combine your inputs with those of others when using XMR? You do. So it's a mixer. We can refer to the other users as "mixers" instead of attributing it solely to the network, if you're concerned about the semantics.
So the cash register at the supermarket is the mixer: I get different inputs in return for the inputs I give them.
Since we're talking about non-KYC services on the forum, maybe it's good to follow theymos' definition of a mixer:
When you put it like that, it's a funny comparison. Criminals use cash all the time too. It's not just the supermarket, it's virtually the whole economy.
And authorities being obsessed only with crypto makes this whole analogy look absurd. But also on the other hand now governments are limiting the legal use of cash. Now in Europe 200 and 500 EUR bills stopped being printed out long ago. Basically every one of these notes reaching a bank is withdrawn from circulation. And it was recently legislated that the maximum cash transaction in Greece can be 500 EUR! I guess other countries are set to lower the limit under central EU directive too.
Looking back at what Satoshi envisioned bitcoin as though, it's an electronic version of cash.
But this alone doesn't change much. It's the fact that it's decentralized that changes everything.
So... As long as the "supermarket" in your example is a registered business, the government doesn't care. They're going to comply with whatever law the government brings out anyday without resisting. As any big business would. Big businesses continue as normal even if we live under an oppressive military dictatorship.
Well, likewise with the supermarket, this forum has become too big to go unnoticed. But we as the users aren't that much of a revolutionary bunch to support it functioning if the government wants to actually shut it down due to it not following oppressive laws.
tl;dr sure gov policy has tons of contradictions if we want a revolution it's not gonna happen on its own