Under this reasoning, it's not illegal to just take things from stores (stores hate this one simple trick). If you're caught and your specific actions are then adjudicated to be illegal, at that point you can just start making a plan to bring the items back (even if some are used/damaged/etc) and everything is fine.
In reality of course, the actions were illegal the whole time. The big festering problem is that there is no actual punishment for government agents who break the law.
The existence of case law / precedent does not affect whether something is "already illegal", but rather only how strongly one can predict if something is illegal. The original tariffs were illegal from day 1.
The point of the analogy was exactly to point at something with a lot of case law where this dynamic is crystal clear (although if Trump starts petty shoplifting after he's done looting our government, it's even odds whether this corrupt "court" will find some way to excuse it. Anything for the cause, of course)
In reality of course, the actions were illegal the whole time. The big festering problem is that there is no actual punishment for government agents who break the law.