So it's okay as long as the player doesn't say aloud why he or she is taking the action? I can live with that. You're cool with it, right?
What would be a reasonable way to you that a toad in the middle of a chaotic battle between multiple combatants swinging weapons, loosing arrows, and slinging spells at each other could inadvertently (from the toad's perspective) get itself killed? The player could just choose to have the toad do whatever it is you find reasonable.
I'm curious what actions from the frog's perspective that it could deliberately take that would inadvertently get itself killed. Wouldn't the most reasonable course of action be to run away? Why exactly is the frog jumping in front of arrows?
Now, the frog runs away and gets tagged in an area of effect spell from someone else? Ok, fair enough. There's no meta-gaming going on there whatsoever. It's not like the frog deliberately jumps into the fire pit.
Like I said, there's no plausible actions that the frog could deliberately take to commit suicide.
C'mon, man - at least get the example right. The character in toad-form isn't making a conscious choice to commit suicide. The player is and, stand-up guy or gal that he or she is, provides a reasonable action declaration for a toad in a chaotic situation.
"I jump in front of Bob's sword" is a reasonable action declaration in your mind? "I leap onto Jane's spear" is a reasonable action declaration? "I leap into the fire pit" maybe works for you?
Thus, the problem. The player is so blatant about this. Now, you've stated that you have no problems with this. You expect players to do whatever is the most advantageous in any situation. I don't. I don't enjoy games like that, nor do I enjoy playing with players like that. I simply don't. Not that it's bad, but, rather, it's bad for me.
On the other hand, I do play with players who will keep at least one eye on plausibility when declaring actions. That's what I enjoy.
Anecdote time.
I DM'd a Scarred Lands campaign years ago. In Scarred Lands, there is a type of undead (whose name I forget now - an elven ghost of some sort) who has a gaze attack. The gaze attack causes you to madly fall in love with the ghost and violently hate your friends to the point where you will try your very best to kill your friends.
With me so far?
So, using the creature in a game, the party wizard fails a saving throw and now loves the ghost and hates the group. Ok, fair enough. The wizard player then proceeds to use every single area attack he had memorized, making sure he hit the ghost as well as the group, thus killing the ghost and breaking the charm, hoping that the group would have more HP than the ghost.
Blatant meta-gaming. It has stuck in my head for years. At the time, I didn't say anything about it. Let it slide, as did the rest of the group. But, that was a HUGE warning bell that this player wasn't long for the group. And, lo and behold, he dropped out shortly afterward, citing play style differences. While no one directly confronted him on what he was doing, I think he knew that the rest of the group had pretty much zero interest. No congratulations when he killed the ghost, no cheering. Just pretty much stony silence every time his turn came up.
So, yeah, the group I play with isn't interested in doing the "most advantageous" thing every time. It isn't fun for us. Had he actually, earnestly, tried to kill the rest of the group, they would have loved it. Even to the point had that turned into a TPK, they would have loved it. Losing can be just as much fun as winning. Certainly as entertaining and interesting.
Playing RPG's where you always try to do the most advantageous thing is boring to me. It's not interesting. It leads to boring stories, again
for me. I am simply not interested in gaming that way anymore.