For my part, what's the big deal about washing melons at the farm? I consider the entire supply chain from farm to table in my thinking and can imagine a number of way stations where the outside of melons can pick up contamination, but I don't eat the rind of a cantaloupe. Therefore, cutting the melon introduces the possibility of having the knife blade carry outside contamination into the flesh of the melon.
For the last sixty years or so, I've been washing off the surface of fruits and vegetables right before slicing. I wash the blade of a knife before using it on another food. Any fruit or vegetable can pick up contamination from the environment or from storage and handling anywhere along the supply chain to the table.
Some growing practices introduce higher likelihood of dangerous bacteria (organic fertilizers) or toxins (chemical fertilizers, insecticides) which are not easily washed off with plain water. The end consumer has no way of knowing whether such dangerous contamination has occurred. A reasonable assumption is to prepare food in such a way to reduce the possibility of carrying contamination from the surface to the the part to be consumed.
The fact these farmers are making a plea deal merely means they and their attorneys find it less "risky" to take a negotiated deal than to take an expensive chance on a trial. It does not necessarily mean their method was illegal, only that it was a bad choice in light of subsequent events, since there is no regulation mandating chlorinated rinse in EVERY growing operation. I'm pretty sure my suppliers at the local farmer's market are not washing produce in specially chlorinated water (other than normally chlorinated tap water straight from the faucet or non-chlorinated water straight from their well.)