So I've only done the SHT from Lutsen to just past Cacade Falls. Along that stretch there are 2 shelters along the trail, one if off the trail a bit, the other is right on the trail near Cascade River State Park. Neither are on the trail map. Neither had any markings or signage saying public or private. Anyone know whats up with these?

Those are state park trail shelters for winter users (primarily cross-country skiiers) to rest out of the wind. They are not on SHT maps because they are state park facilities, not SHT facilities. They are public, but are for resting or emergency use, not camping.

Yes, what Tony said. There is also a shelter on the trail within George Crosby Manitou close to the river.

I've seen people camp by the one up on Lookout Mtn over at Cascade State Park and the one up on the hill at Split Rock Falls SP. Now if it was legal or not is another question. But they were using tents set up next to the shelters and using the shelters for sitting around in and cooking.

The one at Lookout Mtn is a backpack campsite run by Cascade River State Park. The catch is, you need to register (and pay) to use it down at the Park Office (which makes an extra trek -- maybe a couple miles? -- for SHT hikers), and I don't think you will know whether it is reserved without calling the park office or hiking down. Closest water (filter) is at Little Cascade Creek, not far down the hill. Wife and I have camped there; it is a pretty spot. Sees a lot of day-trippers in summer.