This is irresistible:
Why Bicycle Licensing Usually Doesn’t Work CityLab

The idea of licensing and registering bicycles like motor vehicles gets bandied about frequently in the endless debates over whether cyclists are freeloading on infrastructure the Good Lord (or at least, the tax code) intended for cars and trucks when they ride on public roads. (Here’s a good study on why bicyclists are in fact paying more than their fair share to use roads, in case you feel one of these debates coming on.) A handful of municipalities do require such fee-based licensing, however. And it’s not always a complete and utter waste of time, money, and resources. (However, it usually is.) …
Probably the best examination of whether licensing succeeds on any of these fronts comes from … Toronto, where, from 1935 to 1957, bicyclists were indeed required to register their rides for annual licenses, for 50 cents per year. There was a metal license plate and everything, according to this very thorough website from the City of Toronto. The law was scrapped for a delightfully Canadian reason—the fear that licensing “results in an unconscious contravention of the law at a very tender age,” which can lead to “poor public relations between police officers and children.”

Still, efforts to revive velo-licensing surfaced several times in the 1980s and ’90s, only to be batted down by voters. The first and best reason: It would be too expensive to run the required bureaucratic machinery. “If cyclists were asked to cover the cost of licensing, in many cases, the license would be more expensive than the bicycle itself.” …
Another example is Salt Lake City, Utah, where there’s also a state law requiring licensing. “Both are somewhat lightly used,” says Becka Roolf, the city’s bicycle/pedestrian coordinator. She’s not sure how long the laws have been on the books. The license itself consists of a sticker; it costs up to $2, but this local nonprofit will pick up the tab for you. “It’s largely to aid in returning stolen bikes,” she says. “It’s definitely not a moneymaker.”  … And, of course, there’s the inconvenience of losing your license every time you ride beyond the city limits.

The inherent goofiness of this situation, Roolf admits, reflects a deeper disconnect about bicycles in American life, a confusion that continues to foil even good-faith efforts to integrate these devices into our civic fabric. “It’s an interesting dynamic. Sometimes our laws are set up based on the idea that a bike is toy, and sometimes it’s a form of adult transportation,” she says. “As a society, we don’t seem to have worked that out.”