Not trying to derail this thread...................
It's a California thing.....don't know about other states if it applies. I know Oregon and Nevada it doesn't apply.
It's pretty much like this > Any bike 2002 and older can ride year a round regardless of the location, state, county, BLM, National Forest. Green Sticker.
2003 + is a red sticker bike. I think it's 1 June - 1 Oct is limited due to ARB - Areas that are below 2000 feet in elevation, it's a air quality issue. Area's that have closed course competition events, like the Hollister GP track (State owner property) track can run a motocross event and it can be open to red / green sticker bikes. But if you tried to get a red sticker bike at a state riding area like Prairie City OHV during the above mentioned time frame you could be having issues. But......if you take a red sticker bike above 2000 feet elevation, say Georgetown, Elkins Flat ( National Forest ) your good to go!
The air quality here in the valley can get pretty bad due to the heat, ag related and smog issue, so the state figures it can help the air quality be placing restrictions of when and what type ( red or green sticker) bike.
I have both, red and green sticker bikes. The GasGas is a green sticker, the RM is red sticker. The RM I can ride it all year, due to the fact it is closed competition courses, motocross events.
For ME, it's really not an issue, it's just too hot during May to Sept to ride here in the valley. It is much cooler up in the National Forest during those times.
I'm sure Rod can correct me if I'm all wet here, as he know the regulation much better than me. I just ride.......plus green sticker bikes can bring in premium dollars.