Sorry to dig up an old thread. My daughter mentioned this to me today, and I have also wondered about what appears to be the sale of possibly "contaminated" or "recycled" vegetable oil.

None of the answers posted here seemed to make a case for themselves, and were even self-contradicting.

I have a rather plausible explanation for the fish smell in vegetable oils.

In our modern world of commodities for the masses, vegetable oil is vegetable oil. Deli's and restaurants that use deep-frying equipment do a "boil out" on a regular basis where chemicals are dumped into the hot oils, and the suspended particulates drop out. The oil looks and smells fresh if you have done your job well. You may continue deep-frying.
Let's take vegetable oil up another notch- where large corporations use it to prepare processed food products. Hundreds of thousands of gallons of this oil are used on a daily basis. In order to get the most out of the oil, I am sure (but I am guessing) that boil outs are done at this scale, as well. At any rate- eventually the oil is unfit for the product being produced-and probably goes through even more processing before being "recycled" as vegetable oil. This may serve the bottom line well, but the consumers get the short end of the stick.

Just my .02

glowing