They can challenge it, but only via the political process, not a legal process. Congress had been given the power to regulate interstate commerce (lest the individual states try to do so themselves). This wasn't a problem for the first 120 years of the republic, because the government didn't have the budget to maintain a centralized bureaucracy to do too much (nor to mention that the concept of a centralized bureaucracy was born from Prussian authoritarianism that flowered in the latter half of the 19th century).

The nationalistic progressivistic movements sweeping out of Europe hit America in the early 20th century, leading people to ask: "Europe has this, why can't we?" And we ended up with a central bank and income tax all within a decade. Big centralized government built on dangling strands of Constitutional safeguards.

Well, the argument here is pretty specific: the plaintiffs (some farmers) argue that the law is unconstitutional because it seizes their property in a procedure that doesn't comply with the 5th amendment. Only the farmers would have standing to raise that particular argument.

A consumer arguing the law was unconstitutional would have to come up with a different reason, at least.

Right, so the problem is consumers having grounding for a challenge rather than standing?

That's probably not sufficient to rule it unconstitutional. Remember, just because something is boneheadedly stupid doesn't make it unconstitutional.

The farmers going after the government for involuntary expropriation without just compensation have the best claim AFAIK in terms of getting it ruled unconstitutional.

Nobody likes raisins that much.