Also, I don't know many pirate stations that would broadcast solely for the purpose of disrupting another station. My understanding was that most pirate broadcasters do so in order to send their own message, and so finding open signal space would seem to be key.
In this case, KBOO on 90.7 is running HD format, which puts the lower digital shoulder at 90.5 MHz. So this station was operating on top of KBOO, compromising the KBOO signal. 90.5 - 90.6 MHz contained two signals, although the pirate operator didn't know that, as the digital hash sounds like static to the untrained ear.
does KBOO have a right to 90.5 being unoccupied? e.g. did they purchase double the spectrum as those higher-frequency stations?
KBOO did not purchase spectrum as they are a noncommercial station, and those stations are not subject to application or regulatory fees. KBOO has the same rights to the 200 kHz for analog and the additional digital bandwidth on either side, as any licensed FM station in the US has (expect for perhaps some FM translators).
As for analog vs. analog on adjacent frequencies, they interfere with each other, because there's a little bit of overlap. Analog signals don't have that "cliff" that the band edge that digital signals have. Analog FM can fit right next to digital, because the digital signal stops right at the edge. But the little bit of overlap of a 101.3 signal onto 101.5 is doubled when you consider the little bit of overlap from 101.5 to 101.3. It's significant interference on a receiver.
If you look at the lineup of licensed stations in any one location, you'll notice that stations are spaced by at least one vacant channel. For example, in most cities you'll see an alignment like this: 101.3 (full power), 101.7 (low power or distant), 102.1 (full power), 102.5 (low power or distant).
A full power station can't be in the same vicinity as another full power station for 3 adjacent channels. That is probably overkill, as it's made with consideration for receivers made decades ago that are no longer in use.
Pretty much, yes. There are always guard bands allocated, plus careful calculations to avoid harmonics, intermodulation products, images, etc, which could interfere with other services.
They have the rights to what every station on the FM dial has, except possibly FM translators/repeaters: 200 kHz analog signal centered on 90.7, and 100 kHz on each side of that for the HD signal, for a total of 400 kHz. So their signal starts at 90.50 MHz and ends at 90.90 MHz. The 90.5 MHz pirate was operating on top of the licensed 90.7 signal.
(I should note that you're correct about the guard bands with calculations to avoid harmonics, intermodulation products, images, etc)
There a rather complex set of calculations which avoid interference to adjacent stations, plus interference to other services via harmonics, intermodulation products, images etc.
That is to say, is there a license holder for that frequency that is being disrupted or not?
For commons that are not being used, and are not damaged, I think the think reasonable use should be allowed.