I don't think it's a matter of right or wrong thing, it's good to act, either way. My only gripe is that you went in and called the cavalry before even attempting to anyone about the situation. Seems like our country is getting more and more disconnected on a personal level and people can be quick to pull the trigger on notifying authorities. And if you mentioned your own connection in fire and rescue that may have even exasperated things even more, I know it does here when someone on the council or a LEO files a report about anything.
Health hazard? Maybe, in the same way it's a health hazard if a trash bag by a dumpster behind a fish market or restaurant gets ripped open and spread around. I know that us fishkeepers are especially sensitive to what happens to fish, but as far as the laws are concerned they are not on the same level as warm blooded pets like dogs and cats, and don't have all of the same disposal requirements.
I just personally know if this happened at my business I'd be upset that the people didn't come to me first if something like this happened in my parking lot. But I do understand that they have every right to, and don't blame them for it, it's just not how I was taught to handle things growing up.
Then again the owner/workers could've looked at you and shrugged, and at that point I think it would've been fair game. But this is just my opinion and the way I handle things. This is also armchair quarterbacking so being in the situation and analyzing it are two totally different things.
I do wonder what eventually happened to them, you should call and get an update from someone at the store just to see.