Airbnb is suing San Francisco in Federal court over a new ordinance under which the city can sue the company for listing unregistered apartment rentals. The ordinance could cost Airbnb $1,000 a day for every day a nonconforming listing is up. Airbnb says it is just the platform and therefore not legally responsible for the posting of its users, or whether the content is its business.

At the core of the case is the Communications Decency Act, which bars the government from holding internet sites responsible for the content created by their users. Airbnb claims it is just a platform and protected under the CDA. The City of San Francisco disagrees.

“What we’re saying is the only way to enforce this law is to inject corporate responsibility into in by fining the hosting platform,” says David Campos, who is on the San Francisco Board of Supervisors and is the main author of the new ordinance.

There are thousands of Airbnb rental units in the city, and as many as 80 percent aren’t registered. Asking Airbnb to ensure they are registered is no different than renting a car, Campos said.

“Before Hertz can rent you a car, it has to check that you have a driver’s license,” he said.

But Airbnb isn’t renting cars. Nor is it just a few people buying classified ads to rent out extra rooms.

“That’s not very disruptive to anyone, to any status quo,” said Andrew McAfee, co-founder of MIT’s Initiative on the Digital Economy. “Probably not going to attract the attention of regulators in the city.”

McAfee said Airbnb operates on a huge scale. It’s in 34,000 cities in 191 countries.

“These are significant businesses, and they’re reshaping cities and reshaping industries and economies,” he said.

McAfee said the court ruling will probably set a precedent for other cities, and could pave the way for more legal battles.