Is it ridiculous they gave him no leeway? Yeah, sure. But it's spirit airlines, the ticket is dirt cheap so they're going to bleed you in other ways. Let's not act surprised, baggage fees for spirit are nothing new.
The trucks/bearings/wheels are the majority of the $ value of a skateboard.
That board might be old and boards are something that you change regularly... Hence they can be expandable. Plus skaters tend to give one another their old boards if necessary...