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Nothing I post is advice: DYOR.
I very carefuly only mentioned "fine" in quotes. It's a casual label many people slap onto a demand to hand over money. The reality, if you're in a situation where you do not wish to be "held" or accompanied until ID is proved, is that you are usually allowed to post a bail bond, or surety, if you have funds. This is recoverable once ID is produced and any other liablities are repaid.
The surety is held so that, if no ID is produced, funds are available to send you back to the state you claimed to have come from. In an incident, of course, bigger bail bonds are demanded so that likely third party claims can be repaid as well.
If this cash is abandoned, I suppose it could be looked upon as a fine.
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