Supreme Court to Hear Arguments in Landmark Roundup Weedkiller Case

Since 1991, the EPA has held that glyphosate is not carcinogenic; it was (at the time) categorized "Group E", which means that not only is there not evidence for it being carcinogenic, but that there is material evidence that it is not. Later, IARC (in a decision that was controversial among global public health agencies) listed glyphosate as a 2A probable carcinogen, alongside red meat, potatoes, deep fryer oil, and a slew of scary chemicals that includes many other insecticides and herbicides.

States like California enacted labeling-law regimes that key in part off IARC's classification, which meant that in those states Roundup products required labeling. Monsanto/Bayer lost civil cases based on failure to label.

That's the domain-specific stuff. What the court likely cares about is the preemption doctrine. In a variety of different situations, competing state and federal statutes are by explicit or implicit preemption rules. In many cases, federal preemption is a result of bargains with industry: for instance, we got programs like Energy Star after negotiations where industry (and the states dependent on those industries) made concessions to the federal government in exchange for exemptions from state regulation, which is why there's controversy over local municipal ordinances that attempt to ban gas ranges (apropos nothing, but: combustion products of gas ranges: also IARC carcinogens).

There's a weird backstory to public opposition to glyphosate which has very little to do with glyphosate itself (as someone else on this thread pointed out, glyphosate is relatively benign and relatively inert compared other common crop and landscape treatments), but rather with the idea that glyphosate is part of the technology stack of GM crops.

For those people it's worth knowing that the civil liability Monsanto/Bayer is trying to avoid here is approximately the same as the reason Jays Potato Chips bags sometimes have "Not For Sale In California" labeling. Nobody has declared that Roundup is categorically unsafe. Some states have declared that you have to label it the same way you would a gas station or Disneyland ride.

Glyphosate is probably the safest of the things people spray their lawns with. I don't think we should - the worst you get on a typical suburban lawn if you mow but don't spray are dandelions and clover - but it's probably not giving you cancer. As for food... again, there are far worse, more persistent pesticides that escape this kind of scrutiny.