They can drown because they do not understand that they cannot swim through glass.
i would LOVE to know the source of this info (dendroboard i presume?) in fact, before i built my first paludarium for frogs many years ago, i could not find 1 case of a dart drowning. to this day, i still have yet to read one FACTUAL account of a frog drowning. oh, everyone has "heard" of it happening, but there's no proof. i have actually seen, with my own two eyes, Tincs not only jump in the water, but swim under water the length of their tank. i have seen my Mantellas do this as well. in my opinion, it's statements and thought processes like this that actually hold the dart hobby back, not allow it to progress.
my apologies if i offended anyone, i will climb down from my soap box now.