So much for promotional slogans like "Sign up and drive!"

Uber and Lyft drivers operating in San Francisco, a number that may figure as high as 37,000, must obtain business licenses at the order of City Treasurer Jose Cisneros. The Chronicle reported the news first, and Fortune was able to confirm the story with the Treasurer's Office.

Uber and Lyft consistently maintain that their drivers are independent contractors, fighting cases in court that allege they're in fact employees. Fine, the treasurer says: Independent contractors must apply for business licenses, and Uber and Lyft drivers that work more than seven days a year would fall into that category.

Licenses cost $91 each, annually, and if every driver were to register — an outcome that is highly unlikely, as enforcement may prove difficult and the number of drivers who currently operate could differ from that 37,000 estimate — the city would generate $3.37 million a year.

"Failure to respond to this letter may result in penalties and payment obligations,” a letter, which will be sent out over the course of next week, is punctuated.

Though it might seem random, the timing is anything but. Uber and Lyft have long held the names and addresses of their drivers from the city, but now, Cisneros says, he's obtained them all — though he won't say how.

“We have a very broad and comprehensive business registration requirement [in San Francisco[,” Ciseneros said. “This has been a law that has been around for many years. It’s very clearly spelled out on our website — the law here in San Francisco requires you to register your business with the city. If they missed that requirement, they are still obligated to do that.”

The letter is also pegged to new a new online registration system, which launched in March. Previously, it was necessary to take a trip to city hall to register.

“We have serious concerns with the city’s plan to collect and display Lyft drivers’ personal information in a publicly available database,” a Lyft spokesperson wrote in an email to the Chronicle that played up the privacy angle. “People in San Francisco, who are choosing to drive with Lyft to help make ends meet, shouldn’t have to compromise their privacy in order to share a ride.” Lyft has stated that it is opposed to the plan.

However, Uber isn't putting up a fight. “Uber partners with entrepreneurial drivers and as independent contractors, they are responsible for following appropriate local requirements,” a spokesperson wrote on behalf of that company.

Mayor Lee's office says that the decision rests not with them but with Cisneros. "The mayor defers questions about the definition of independent contractor and interpretation of requirements under city law to the treasurer,” Lee spokesperson Christine Falvey said.

On Uber driver message board Uberpeople, one driver, dbackagain, speculated that out-of-town drivers coming in to the city proper to collect fares might shy away as a result of the change. "The part-time soccer mom and dad will have to re-think about coming in to SF for the 1st time and ponying up $100 bucks for a license," they wrote.

Another driver located in San Francisco, Uberdancer, wrote "Oh, good ... does that mean drivers from the East Bay, South Bay, Sacramento, Davis, Stockton, and Marin will stay away from San Francisco, the most photographed city in the world where the women are stronger and the men are prettier???"

Time will, tell, Uberdancer. Time will tell.

All previous Uber coverage on SFist.