new players post small amounts of food due to the fact that it is the most profitable early income source without any infra investment on the base world. So if they are a new player or are just starting up there 1000th empire, it`s fast money.
Also if they are an established empire, usually tax based or eating a lot of food, they will spam the market with cheaper food to tick people like you off. The big food producers get fustrated with spending 20 minutes buying up small batches of food and then end up just doing a large sale at that price or cheaper. The batch seller than just watches the market and skims off the cheap food and makes more of a profit from their tax based income or just for a simple re-sell for fast profit.
I was at one point a major food supplier (27k U. Fert will do that making oer 2.9 mill food a turn on RT..as several vets for confirmation) and I did that, drove the food market down to 4 per for several days and have tens of billions of units of food sitting at 8 and 9 per waiting for the market to bounce back up, skimming a cool 200-300 billion profit for myself along with whatever was made by my colonies at the time (1 billion food per 30-40 minutes if that near the end).
So it`s to tick peoplelike you off and to play with the market for massive potential profits, and massive potential losses but with bugger all to really spend hundreds of billions on and sitting near max cash...nothing better to quickly burn creds...