Is it surprising though? Your consumption is an important part of any market. By removing your demand from the market, you're affecting it and the price of wheat or whatever good. At an individual level, it probably won't change the scale much, but if enough people do produce only for self-consumption, it can have large enough effects on the market that there shouldn't really be any question that it falls under commerce. The question should rather be whether to regulate the market for wheat at all and what incentives/consequences that might have.