Well, its just $$$'s, passport loss could become a very tricky complicated issue. The good ol' never-ending stories about bike rental shops around the greater Asia region and passport scams... Pay up $$$sucker$$$ or we keep your passport, people (vehicle renter) really still that desperate and naive to hand over passports while engaging with vehicle rental agencies ~ makes me wonder seriously... Ride on!!!

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Somewhere down the road in China since '89 ~ along the route I've learned the hard way that pleasing everyone is impossible, but pissing everybody off is a piece of cake....TBR

Why don't you guy start up a motorbike rental company?
And just put the shoes on.
There you are with all your new bikes to rent out to a person you don't know and you would like some damage waiver money or a passport for security.
How out of the way is that to ask.
So leave your passport or leave some money.
We ride our own Motorbike with respect, do you do the same with a rental?
I am involved with a big Motorbike Rental shop in Thailand with over 200 bikes.
And I can tell you first hand how some of the bikes are returned.
Even if you think you are smart and leave your passport as the deposit and you crash the bike and don't return it, thinking shit I just apply for a new passport and tell your country embassy I have had my passport stolen or lost?
After you are 1 day late returning the bike, we then Email your local Embassy of your country we are holding your Passport as security?
Guess what now you are committing fraud by applying for a new passport.
Has happened a few times now with the end result being you have to come back with the bike.
But now you pay the full price to fix the bike with no favours own end.
Your ass will be kicked.
So play the game by the rules and you will be ok.
Do your research on the hire company for feedback and reviews and you will be fine, or just WALK on.