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Technically, it's more like a cabin.
As for why they are protesting, well that is an interesting story that goes deeply into the Bureau of Land Management's efforts to expand the Malheur NWR by strong-arming the locals into selling their land. The tales are interesting if you take the time to look them up. If you believe their side, the short story is that these guys are serving 5 years for two "arsons." The first was an approved controlled burn that got a little out of control with no property or human damage. The second was a backburn to stop a naturally-caused wildfire that saved property and possibly human damage.