It isn't?

The only two other federal countries I can think of off the top of my head that has different criminal law in different subdivisions are Australia and Mexico.

In Canada, criminal law is unified on the federal level. So far as I know, this is also the case in Germany and Switzerland.

Any other prominent examples?

It looks like the subdivisions of the UK also have their own criminal law (e.g. https://en.wikipedia.org/wiki/Scottish_criminal_law )

Ah, good point. Although the case with UK is a bit unusual, in that all those powers are still devolved from the British Parliament (and so it could rescind them with a simple majority vote in theory... though that would surely trigger a constitutional crisis now) - so technically it's not federalism.

In contrast, most federations designate who has the power to enact criminal law in their respective constitutions.

Yes, the federal system reserves more rights for American states than are enjoyed by, say, French régions. They're still a far cry short of sovereign states.