Nasus gains Siphoning Strike stacks upon killing Illaoi tentacles

While playing Illaoi vs Nasus top lane i noticed he was gaining stacks by killing my tentacles. I don't know if this is a bug or intended bgut it allowed him to be hit by my e and avoid the damage by running away. The tentacles that spawned b/c he was a vessel gave him the ability to gain stacks safely and easily without worrying about harassment. Shouldn't this be fixed as his ability description says "If Siphoning Strike kills its target, Nasus permanently gains 3 stacks of Siphoning Strike, doubled to 6 stacks if the victim was a champion, large minion or large monster."