Yes, moral grounds, I'm sure. But there are all sorts of abhorrent treatment of animals - abattoirs, indoor farming, force feeding, etc, etc - so, why is it a moral line to drown rats in the way described? Who decides this stuff?
You can do a scientific study without torturing the animal - if you have snow alternative, please explain clearly why, why you study is important enough, and convince the ethics board.
You can't do a drowned rat study without drowning rats.
You can eat all kinds of things without torturing the animals - if you have no alternative please explain clearly why, why eating meat is important enough, and convince - actually absolutely nobody, nobody in power cares about animal suffering in our food system.
And before you ask, no not everyone has to eat meat, but, there is at least the reason that an all plant diet is intractable to some people for the sheer amount of carbohydrates, and for others due to the mental load of making sure they get the nutrients that are hard to get in plants but easy to get in meat.
I'm not a vegetarian or anything just pointing out how nonsensical the above post was. Your take seems to be that it's a matter of ease or convenience that we eat meat - I think that's more or less true.
But it's probably also more convenient to do drowned rat studies to figure out specific things than it is to construct more elaborate ones to ascertain the same kinds of findings. So I don't think my take is that far off the mark, still.
You're asking a genuine question but careful not to sink too deep into whataboutism.
Just because abhorrent treatment of animals exists in one industry doesn't mean it's OK for it to appear in another, or in this case, in the field of science.
As you can imagine, there isn't a single combined committee for these two...