I think the main reason they don't have a Linux player is that their current offering is based on Silverlight.
I guess their thinking is that the technically inclined use Linux, and if a Linux desktop client exists, someone will get bored one day and reverse-engineer the DRM. As it stands, you have to have significantly more determination than that to crack the DRM from Boxee, and it appears that nobody in the community has that determination since we have not seen a crack or reverse implementation.