So, just to answer this in two parts:
As a poro cuddle is the closest thing we have to an expression of True Love, doing anything but true damage would be at best questionable.
…From a slightly more mechanical perspective: the poro throw spell’s “original” incarnation (or the closest thing to it) was actually a mode where you had two damaging summoner spells: The Poro Throw, which was basically the spell you have now, and “poro lob”, which we ended up using for Poro King’s friendship barrage. In that one, the throw was doing physical damage, and the lob magic (and we were throwing around ideas for making them scale on AP vs AD, etc.). What we quickly realized however was that one of the spells always felt “weaker”, especially against tanks: I would buy a last whisper, so my throw would be awesome, and my lob would be somewhat meh, or the reverse with Void Staff.
So, to make it feel like it was doing good damage regardless of the target/ your build, we decided to make the throw do true damage once we moved to just having the throw. The full extension of that would have been making it do percentage of max health true damage, but that might have been a bit much :)
As for the second question:
In general, our goal with the poro throw was to make the players feel “I want to throw it as much as possible”, because throwing poros is, in general, fun. Adding an incentive on death somewhat changes the decision making ,and we felt it would have one of two outcomes:
Case 1: If the incentive was very strong (Think Darius Ultimate), players would want to use their poro throw less, and “store it”. Not a bad thing necessarily, but not what we were aiming for.
Case 2: If the incentive is weaker (Say, “Get +1 gold”), it wouldn't actually really change your behavior, so would just be additional information that could be a “trap”, as you try to optimize around it even if you shouldn't.
So, basically: We want you to throw poros as much as possible, and were concerned that adding an on-death incentive could move your incentives away from doing so.
