I don't understand how he didn't get into trouble for what he did. Why wasn't Secret Service able to prosecute him?
The only reason USSS got involved to begin with is because the casino security officer was (rightly) suspicious of some guy in the casino ripping bills off a pad and feeding them into a slot machine...
Also, I don't have a lot of casino security experience, but if a guy rips bills of a pad, it seems more likely he is joking around instead of counterfeiting. If you were counterfeiting currency you wouldn't want it on a pad, that's not very realistic at all.
Currency protection is one of the absolute most critical functions of a government, if people lose faith in your currency it causes a ridiculous number of problems.
When the US was young it's presidents didn't require the level of protection they require now (in fact a few of those presidents would have been as likely to shoot an attacker as not).
Eventually they realised that presidents do require protection but deciding who to use for that protection is quite a nuanced task, there are risks involved in having military protection or police protection (remember the political climate at the time was one of ongoing revolutions around the world and in Europe) so they needed an agency that existed, was extremely professional and likely wouldn't ruffle too many feathers.
The US Secret Service fit the bill ideally.
Giving a fake ID to a secret service person might be illegal, but at that point they might have figured out he was pulling their leg. Even government officials sometimes have a sense of humor.