Can an Airplane Passenger Ignore the "Fasten Seat Belt" Sign?

I was on a plane recently, listening to the usual safety briefing, when I heard the flight attendant say that “it is a violation of federal law” to ignore illuminated safety signs, such as the “fasten seat belt” sign. I was surprised because, on another flight, I had overheard a flight attendant tell a passenger who wanted to use the bathroom while the “fasten seat belt” sign was illuminated that she couldn’t authorize him to get out of his seat but that she wouldn’t stop him either. The sense I got from that previous exchange was that the sign was essentially a recommendation. So, I decided to look into it. It is a violation of federal law to ignore the sign. 14 C.F.R. 121.317(f) states that an airplane passenger “shall fasten his or her safety belt about him or her and keep it fastened while the ‘Fasten Seat Belt’ sign is lighted.” The statutory basis for the rule is 49 U.S.C. § 44701, which allows the FAA to issue “regulations and minimum standards for . . . practices, methods, and procedure the Administrator finds necessary for safety in air commerce.” The violation is civil, not criminal. The applicable penalty provision is 49 U.S.C. § 46301(a)(5)(A), which authorizes “a civil penalty of not more than $10,000” for violations of “a regulation prescribed . . . under” 49 U.S.C. § 44701. Enforcement is extremely uncommon. This Slate article reports that “in the last five years, the [FAA] hasn’t taken any legal enforcement action against [...]