Something you sell is, generally speaking, more vulnerable to confiscation than something you make yourself... I mean, taxes, for example, are generally levied between when you sell a thing and when you buy food. (in my jurisdiction, I don't have to pay sales tax when buying food I intend to eat, but if I sold something to afford that food, I'd have to pay taxes on that income. If I grew food for my own personal use, there would be no taxes involved.) For me? It's not a huge deal because food (and housing and other post-tax expenditures... ) is a very small portion of what my business brings in,(most of my expenditures are things like electricity and payroll, which come out of the business pre-tax) but i imagine it makes a difference for sustenance farmers.

I mean, I'm not saying taxation is the only way it's easier for others to get their hands on your crops if the crops are bought and sold... but it's one of the more clear examples.