Very interesting question. My question to you: "Why do we call it a 'Fire Engine' when it actually does not carry fire? Does it trigger you to wonder why?
I feel that 'FIRE,' 'AAG,' 'AGNI,' is a word easily associated with the intended purpose of a 'Fire Bucket' to extinguish fire. A signage is used for easy identification. Diplomatically speaking, it is as per ISO Fire Standards.
I feel it would be easier to write with bold letters 'FIRE' than to write 'To be used on FIRE' or 'Aag Bujane kay Leya'.
Regards,
ukmitra