Because with Nitori, you get more than what you paid for.

Considering that she stole it, I'd say that'd be accurate even if it was a normal broom.

Perhaps Nitori put that on there in the event someone stole it.

Come now, Marisa, one can't call herself a proper (M.A.D.) scientist without installing a self-destruct mechanism in every one of her inventions, can she?

You of all people should know that.