1) “Spam is a low-quality page that ranks above a high-quality page” works pretty well
"Quality" is too subjective for your purposes.
2) Even a quality result is only quality the first time it appears in the results. The repeats are all Spam.
Let's rephrase this: Even an authoritative result is only relevant the first time...
3) A page with stolen content is always Spam...
Nope. A page with stolen content may be a great resource. Just because it's infringing on another's copyright doesn't make it spam or junk. Especially if the content is stolen from a real authority.
I don't think it's up to the SE's to decide what's "stolen" and what came first and all that. Honestly, I don't think they're capable.
Bottom line is, if I search for "widget clips" or "widget wmv" I'm not likely to get the official site of the film Widget, nor will I get the studio or anything official or authoritative.
I will get a bunch of pages with stolen, literally illegal and pirated clips of the film. Some will just have the trailer scraped from the original widgetthemovie.com site (which was not optimized at all, since it was an indie movie and there was no budget for it).
Of course, this stolen, illegal content is exactly what I wanted, and that, as a user, is not spam.