So instead of the tahm getting to use the ability to save an ally who doesn't realize he needs saving, the ally can get themselves killed and the tahm wastes the cooldown. Trust me, there is a reason Tahm can make the choice. When your Caitlyn thinks they'll get that ult kill on the syndra because she's low, and you the Tahm notice a jax getting ready to jump over, along with two other members of the enemy team closing in, you want to be able to FORCE the Cait into your belly so they don't die, not them choose to stay out and get killed because they didn't notice that they were about to die. (That Cait chewed me out btw because "I could've killed her" and then told me to never use my W again ever.).
Sure, Tahm can troll you with it. So what. When that shit saves your life because you didn't realize you were about to die from a gank, or you made a bad move, or pretty much did anything that could end poorly for you, you'll be happy you got the instant help, rather than die because you didn't click on the Tahm because "you didn't need any help". Tahm get's flamed enough as it is, even when he saves your ass. He don't need to also get flamed because "he didn't W me" even though he tried and you didn't click him to get consumed.