Actually, the logic isn't difficult to follow at all.
The body is in a constant state of flux. Sometimes it's low on potassium, sometimes it's not. The kidneys filter out compounds not in need at the time they do the filtering. At a later time, those compounds may be needed by the body.
The purpose of the kidneys is to maintain systemic balance. As such, they regulate acidity, salts & minerals, and blood pressure. However, they DO, contrary to what someone else posted, filter out some waste products as well -- waste such as ammonia, urea, and by-products from drugs the person might be taking. This is in part why people with kidney failure have to go in for the weekly filtering.
With that in mind, I don't believe urine is a solid product for consumption; however, as we've seen with psoriosis, different things work for different people.