If the couple had left it in their account and returned it when asked, there would not have been an issue.
If you earn 3k a month, one month you get paid 100k, and you spend 95k on stuff you don't normally buy, you are going to be arrested.
If you pick up 20 centrs from the floor and spend it, you won't be arrested.
If you find $50k in a bag and spend it, you will be.
"the person saving or finding the property shall, if the property is of the value of one hundred dollars ($100) or more, within a reasonable time turn the property over to the police department of the city"
The UK has a similar system for fines. Laws are written that if a particular offence occurs, a "Band A fine shall apply", and a separate bit of regulation says what each band of fine or punishment entails.
Sure, but we all know that never happens until a sufficient number of easy to feel sorry for people get screwed real hard. Those people are getting needlessly screwed because the legislators were lazy. For example, most states have felony charges as an option for vandalism over a certain dollar amount, in some states these dollar amounts are low. This results in teenagers getting threatened with felonies (and the charges inevitably stick sometimes) because cleaning up their mess cost a few hundred bucks. That is a level of draconian-ness that is not ok in a free society.
If you find $10k in cash in a bag
If you find a $10k car parked on the street
If you find the latest iphone sitting on a table
I get what you're getting at but I'm saying that there are plenty of details that matter regardless of ease of proof that the object isn't yours or whether or not it's foreseeable have been left there under "reasonable" circumstances
Cars and phones are trivially easy to track back to their owner, with cash possession is basically ownership. Cars get parked places, phones get left places, cash does not get left lying around and if you leave cash somewhere you basically forfeit ownership of it since that's the cultural norm since ownership can't be proven.
The specific facts and details matter very much. Time and attempts to find the property by the owner are particularly important. There's a very big difference between leaving your phone at a bar and leaving your phone at a bar for a week, in the latter case the bartender has a nice new phone they can use.