I would say the important factor mentioned in the article is that heavy metals from the batteries will end up in the waterways if they're not recovered. A bunch of drones crashing on land an be a) (presumably) recovered much easier and b) even if you don't find one of the drones, it's probably going to take a lot longer to find its way into the water and it should be in lower concentrations etc.

mvdtnz' assessment of the Yarra being full of garbage already is correct, though. It wasn't too long ago that Melbourne locals were intentionally throwing hire bikes into the river because they were sick of them taking up the side walk. I think it was numbers well into the hundreds of bikes that were being dredged up.