Chris, I remember you from the ZRXOA board. Didn't you race with WSMC years ago?
I retired from Supermoto racing a few years back so my info might be out of date.
The kart track at Prairie City (and some other facilities at the park) are leased to clubs or businesses. The club renting the kart track may not allow motorcycles due to their insurance or just being fully booked with karting activity. That has been the case historically. Some vendors have been more moto-friendly than others. Its really nothing to do with a pro or anti moto bias, just the popularity of karting and/or changing insurance regulations. Thats why SupermotoUSA doesn't race there very often.
I only raced at Redding once but loved it. Really fun layout and I know its been repaved since I was there so its even better now.
Sonoma is a great facility with several excellent track layout options for pavement practice. The surface is good but the asphalt is very sensitive to weather conditions. Full dry is great, a little moisture and it's like ice. Also, during winter months some parts of the track are in the shadow of the hills late in the afternoon which causes track temperature in those shadows to drop very quickly. It's like a light switch how fast it changes.
Old-man-shakes-fist-at-cloud-warnings aside, Supermoto track days are ridiculous fun. Have a good time.