First off, I hate the term "synthetic nutes" (sorry Jbone). When using soil, bacteria need a source of NH4+ they can convert to NO3-. Plants can take up both NH4+ and NO3-, but greatly prefer NO3-. The nitrifying bacteria regulate the NO3- level. When it gets to high, it slows down (nobody likes waste).
The reason it makes no sense to use hydro nutes in soil is because hydro nutes provide almost 100% of the N as NO3-, thus there is no food for bacteria and no reason for them to exist anyway. If you feed your soil with 100% of your N as NO3-, you may as well use a hydroponic medium, because your soil is already lifeless.
As for these welfare growers, I can grow cheaper with hydro than they can grow with dirt. Calcium nitrate is about 2-3 dollars a pound. Indoor dirt is a luxury when compared with hydroponics.
Piss is high in urea (go figure) and other minerals, so it could theoretically be used to provide nitrogen, but like i said, this urea would need time to broken down and used by bacteria to get the NO3- level up.