I disagree. Just because someone offers something does not mean it is safe. That is a very naive attitude that get's people killed as it turns out........
The very fact that a dive operation offered this type of dive implies that it is safe for divers who act reasonably under water.
As another example, consider boarding a plane during bad weather. You expect the pilot to have your safety in mind and not take off if it's dangerous to do so. If he does take off in dangerous conditions and the plane crashes because of that, it's the pilot's and airline's fault; the passengers can't be held responsible because they should never have boarded a plane that might have taken off in dangerous conditions.
Passengers have a reasonable expectation that an operation (diving or airline) has their well-being in mind.
Boarding a plane in bad weather is not the same as diving. The limit lies with jumping off the boat and doing something yourself where the 'pilot' has no further control. I agree that when you board a plane you can expect the decisions for your safety to be made by the pilot. The same goes for the boat, you expect the captain to keep you safe. But the moment you jump over board to dive, the captain has no influence (whether you want him to or not) on your safety. You are doing something dangerous on your own as an individual.
Throw chum and sharks in the mix and you have the makings of a disaster.
As I said before, the operator should not add to the dangers of diving by chumming waters, but I would not go so far as to hold the operator completely responsible. It is irresponsible to offer the chumming trip, it is suicidal to actually sane of mind and freely jump in the chummed water.....