All these guys are trying to say is: the cause of Psoriasis is dietary and lifestyle based. If you have psoriasis, regardless of where it shows up, you're doing something wrong in your life, and you have to find out what it is in order to get rid of your psoriasis.
Treatment, from the perspective you mention, is just a means of masking the symptoms of the underlying problem. For that, yes, it is handled differently. To treat scalp psoriasis, traditionally you would use a shower cap and various creams from steroid-based to sulfer-based to zinc-based BUT this does not solve the problem of WHY you have psoriasis.
To figure out why you have it, you need to begin cleansing your system and eliminating various foods until your symptoms go away. Then add back in until your Psoriasis returns. This helps you understand what causes it in yourself.