"But the city's goal with these sting operations wasn't to catch and prosecute drivers/uber for previous trips."
Errr, sure it is. Their investigatory mechanism happen to be to try catch them on a trip they could prove originated in the city, giving them probable cause to stop them and ticket them.
This just happens to be easier to do as a sting.
"It was to catch them in the act for a single trip and prosecute them for that."
That is just the method, not the purpose of the investigation.
Your argument is what would be called "a distinction without a difference"
If the FBI is looking for illegal uranium sellers, and they try to buy/bust a low level guy as part of the investigation, it does not make the investigation about the buy/bust, it's still to catch and prosecute illegal uranium sellers.
Here, the investigation's goal was to catch and prosecute uber
drivers driving illegally.
In any case, it doesn't matter.
Remember "Obstruction may consist of any attempt to hinder the discovery, apprehension, conviction or punishment of anyone who has committed a crime. "
Not "the crime you are currently investigating", but any crime.
For example, oneo the the federal statute says:
"(a) Whoever willfully endeavors by means of bribery to obstruct, delay, or prevent the communication of information relating to a violation of any criminal statute of the United States by any person to a criminal investigator shall be fined under this title, or imprisoned not more than five years, or both."
(emph mine)
(Ignore the bribery part, there's a ton of these statutes,i'm just pulling out one)
"To use another analogy, if I'm a drug dealer and the cops try to buy drugs from me in a sting operation and I somehow figure out that they're cops and walk away before doing a deal is that obstruction of justice? Because I think that's basically what is (was?) happening here.
"
This isn't obstruction, but it's not what is happening here.
Imagine if the cops were investigating you for dealing drugs. They already had evidence you deal drugs. They ask your friend where you are, even just to talk to you, to try to get cleaner evidence, and he deliberately lies about where you are to protect you. Congrats, that's obstruction in a lot of jurisdictions. No sting reuired.
Here, the cops are asking the uber app where the drivers are. They know or have probable cause to believe the drivers have committed crimes (again, the sting part is just about having a very clean prosecution, it's not required or necessary here).
Uber deliberately lies about where the drivers are, with the purpose of protecting them from this investigation.
You can ignore the sting they were doing completely. The crime was complete the second a law enforcement officer, trying to investigate the crimes of an uber driver, opened the app in an attempt to find a driver (even just to talk to!) and got fake results.