Why not? Isn't Australia free to not allow Facebook to operate in their country if they don't pay a fine? And if the US prevents them from doing so by retaliation or sanctions, that would actually be closer to warfare.
A consumption tax is a pretty good way to tax foreign companies though imo, as long as your domestic market is desirable.
What happened with China?
I know FB isn't really operating in China. (At least no one I knew in Ningbo had it?) So did we sanction China for that or how did that all work? Because, and I don't know if we did or didn't, but if we didn't sanction China, why sanction Australia? Was that situation different somehow? (Other than just "China's richer").
So? It's about power, the US has it, Australia doesn't.
Australia can fine them a hefty sum, but nothing that looks like an existential threat to the company.