Why doesn't "Scorched Earth" do what it's supposed to?

[ShrubberyJSC]

The opponent casts "potion of heroism" on a tiny goblin.

I know from past experience, this makes the goblin pretty much unkillable by my creatures, so I play "Scorched Earth"

The card says "Playing this card kills all creatures and removes all corpses from play."

So why doesn't it kill the stupid golbin?

The "potion of heroism" doesn't give it protection from spells, it just gives temporary HP.

So, WHAT'S UP WITH THAT???

[Acacea]

As seen here, it makes them immune to one scorched earth or assassin, the next will work.