I agree with the first part, and that's the problem. Basic chemistry is like attracts like. Think about degreaser's, they are oil based because they attract oil. My favorite degreaser is a banana oil product, it attracts grease and as a bonus smells good. Anyhow, you are right I would not put a petroleum based product on a petroleum based product.
As for second part, Most of the good care car products are water based with petroleum solvents that leave the product after application. If you use a petroleum based product the oil does not leave and and the surface stays oily. Funny you mention megs because I do have an oily issue with one of their products but I will leave that nameless, those who use it know it.
Now is this exact? No. Everyone blurrs the line because there's a million different solvents and many different levels of petroleum in any given product, and some solvents may not completley dissapate, or leave a residue, etc. I have used probably twenty or more different surface products and there have been a few that leave the surface oily and I don't prefer them. Open a bottle of powershine and it smells like crude oil, equals petroleum based, I have a bottle if you promise to put it on your car I'll send it to you cause I aint using it. Would any of you let a dab of motor oil dry on your car for a month in summer? I wouldn't put vasiline on my car either. How about filtered water? Sure no prob, let it dry baby.
I'm not dishonest and disingenuous, my favorite sealant, cleaner, and topper have petroleum distalates, but it's not petroleum based, and I think that was the first dudes question.