"Illegal contracts" is a misleading term here. These are not contracts that are illegal because they stipulate some action that is ipso facto criminal (like e.g. an illegal drug sale contract) or because they stipulate a transfer of rights that is inherently unenforceable in the existing law (like e.g. an indentured servitude contract). Rather, the issue is about perfectly normal and ordinary transactions that just happen to run afoul of the law in some relatively minor way, as in the given examples of ordering a meal in a restaurant that stays open beyond its licensed hours, or hiring a gardener who doesn't report this income for his taxes.

The relevant questions here are how severe such violations have to be to void the contract altogether, and how eager the government will be to prosecute the violators if this information comes out when a breach of contract is adjudicated in court. Obviously, in any legal system, both issues are a matter of degree, and clearly different countries will have different systems, with Germany apparently being unusually lenient on both counts. With this in mind, I fail to see any relevance of this fact for my above cited argument.

Germany reportedly enforces illegal contracts.

Irrelevant. The question isn't whether the state refuses to enforce all illegal contracts but rather if it refuses to enforce some; no state enforces all illegal contracts.

Most jurisdictions in the U.S. enforce some illegal contracts. It depends mostly on the comparative culpability of the parties and the importance of the public policy making the contracts illegal.

Though presumably only 'grey market' contracts are being enforced.

I imagine any attempt to enforce, for example, a slavery contract while maintaining illegality would lead to international and continuous outrage (among other things). The degree of social proscription is too strong.