That flat spot is either a defect or a result of poor storage.
If it is just storage, then heat should help. Maybe a week or two in Phoenix would do it.
Maybe a hair dryer would warm up the glue to help it release.
This won't clarify the law in California, but Washington has this Note in the definition:
Finding—2015 c 145: "The legislature finds that at least two companies in Washington have developed a one-wheeled device for people to use to travel from place to place. These devices are unregulated and can travel wherever and however they like. By adding these devices to the definition of an electric personal assistive mobility device, they become regulated and local communities can determine locations that are appropriate for their use." [ 2015 c 145 § 1.]
Some places have special rules for Electric Personal Assistive Mobility Devices, which I would argue applies here. These regulations were designed for Segways
Washington state law: http://app.leg.wa.gov/RCW/default.aspx?cite=46.61.710
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