Straight from Seapearls website-
Exterior of cylinder is first coated with zinc and then finished with a white baked on powder coat finish for years of great looking corrosion resistance.
ZInc is just another word for galvanizing. In the case of Worthington, I beleive the technique used is the "hot Spray" method instead of the "Hot Dipped" method. As to why are the tanks painted, the answer is they are not, they are Powder Coated. If you'd like to know why they are powder coated, this website probably will get you close to the answer:
http://www.powdercoatingonline.com/html/PowderCoatingoverGalvanizing.html
Keep in mind that the industry knock on "Hot Dipped" cylinders is that they are more prone to internal flash rusting problems because of the "Hot Dip" process.
So you can either A) live with some occasional external rust that you can see and take care of right away or B) you can take your chances with the other process that might leave you in the position of needing to tumble your tank every other year because of internal flash rust problems.
Regards-