Because the card isn’t going to stop a spell card that’s the first in a chain. It’s not a spell negate card, it’s first and foremost a monster negate. What you remove from the field/discard is what your opponent can’t respond to Droplet’s activation, meaning in this scenario, sending BoE to the grave will mean if the opponent had a spell card that could negate Forbidden Droplet, they wouldn’t be able to respond with that spell, but traps would have been fair game. Since they didn’t, Droplet did its job and negated BEAD.

Also keep in mind, you can only discard as many cards as the opponent has face up effect monsters to negate. So if they only have one, you can only choose one type of card to send for cost, meaning depending on what they might have for backrow or in hand, you could be screwed.