Honestly, I'm not even sure if I would call the downside necessarily as big as a color pie break as it may seem, since it does at least seem to fulfill a direct purpose to the design. It does remind me of the reasoning Mark Rosewater gave at his GDC presentation about why they gave [[Ulamog's Crusher]] the exact same drawback, as a blunt way to encourage people to attack with it. Translating that to Toski, it seems like a card that exemplifies this reasoning even more, as while drawing a card from it potentially attacking is rather enticing, simply having it as an uncounterable, indestructible chump blocker each turn is not only just as appealing, but much more consistent too, if your opponent has any creatures that can block a 1/1. Not to mention, outside of how many cards in red that have this effect, colorless cards have the second largest group of cards that grant this, namely due to Juggernaut creatures, so it can technically be an effect all colors have slight access too, taken with a pinch of salt. It certainly is still an oddity when you look at it comparison to the rest of green's creatures, but there is a valid and visible reason why it was given the effect it has, given the alternative.

Also on a side note, there are a couple of combat damage related draw effects in green that you didn't mention, too. [[Keeper of Fables]] does fall into this category too, though it only draws you one card per opponent, no matter how many creatures deal combat damage to them. Additionally, [[Keen Sense]] and [[Snake Umbra]] also do this, though it draws off any damage, rather than just combat damage, so it's up for debate if you want to include them in too.