The "blues" created the legislation & forced the workers to accept a shitty contract with no sick days. They could easily have refused to vote for it and let the strike happen w/o real sick days.
Umm, if everything was built around the idea that tweets were immutable, yeah I could imagine it's quite a bit of work to change things. Immutable and mutable data can be treated _very_ differently in systems bigger than the little toys you make for fun.
So... unless Grinnell does things very differently, the author did not learn Racket, they learned (Beginning/Intermediate/Advanced) Student Language, and that's the point.
Racket is a complicated language, designed primarily in order to support the easy creation of other languages: it would be as bad a choice (perhaps worse) for a first language as any other. All of the simplicity, focusing on learning to program, programming structurally based on data, that comes out of HtDP, is enabled by the restricted language.
It's too bad that this naming confusion persists, as I think it hurts the effort to focus on teaching _programming_ in intro classes, vs. teaching X language (I don't want to teach people Racket any more than I want to teach them Python or Java. They can learn those on their own -- they'll probably learn at least a half dozen other languages over their career, all on their own, if they stick with it). This curriculum is about figuring out how to most effectively teach people to program: the language was created, after the fact, to support that.