Well if legalities are not a concern for you then perhaps you would support kidnapping and slavery as an answer.
I say that legality isn't the only concern. You say that I don't have any concern for legalities. Thats a blatant straw man.
Kidnapping is a good example. If a person is kidnapped the only concern is not to capture and punish the kidnapper but to do so w/o getting the kidnapped person killed. It would be considered by most to be foolish and irresponsible to have as your only concern, or even primary concern, the apprehension of the kidnapper. Likewise, when speaking of farmers, it would be overly simplistic to hold up legality as the only concern. Its no more reasonable than insisting the police to capture the kidnapper no matter what happens to the kidnapped.
So, if hiring legal workers could mean the ruin of their business, insisting that its all about legality is...unreasonable, at best. And to insist that they play by free market rules when neither the supply or demand ends of their business operate by free market rules is inconsistent.
If the consumer won't pay the price - then you either change your product, your process, or innovate in some other way.
Insisting on innovation and change by the business owner when wages are artificially high and prices artificially low due to government interference in the market isn't very capitalistic of you

I agree that a legal free market system would be best, but the free market system doesn't exist in any real terms. The American worker has come to expect artificially high wages and the American consumer has come to expect artificially low food prices that innovation and change by the farmer is not longer a "free market solution". Its likely to result in the death of an industry w/o any realistic expectations that it will be replaced by an American solution.
Breaking the law is not the answer.
Neither is the answer as simplistic as you make it seem. For instance, if American growers go out of business because they can't afford the legal labor, then that makes it more likely
- that jobs move out of the country altogether
- American buyers purchase more foreign goods
- other related industries suffer as well
In short, innovation and change may require a major upheaval that, in the end, is more harmful to American interests than letting things remain as they are.