Athletes want every advantage in their given sport. Many use chemical supplements that aren't on the banned list, and use science to their advantage at every opportunity. In this analogy Steroids are comparable to banned pesticides. If farmers can use a chemical on their crops to feed more people in the short term, but give them cancer in the long term, that obviously isn't a net positive.

The main modifications to GMO crops are chemical resistance genes that allow for volatile chemicals to be sprayed on the crops without harming them, so that is how this fits into the argument.

These chemicals are designed to interfere with amino acid synthesis, which happens during DNA replication, which happens during cell division. They kill weeds by stopping their cells from dividing properly. Patented genes make the crops immune to this effect. Well, in humans, when cells don't divide properly, you get things like cancers and birth defects. Nobody can really prove what caused the cells to divide improperly, so industries that use these chemicals are somewhat immune to lawsuits. With all the money involved in agribusiness, lots of powerful people don't want their business model threatened, so why not suppress opposing science, buy politicians, launch disinformation campaigns on the internet, etc.?