Why is the sale of prohormones allowed in some countries and not others?

In countries where prohormones are sold (USA, Canada, United Kingdom...), prohormones are sold freely due to their misleading composition. Legally speaking, prohormones are not anabolic steroids 

since they contain an inactive compound derived from an anabolic steroid. But during digestion, the prohormones are well absorbed by the body like a real anabolic steroid...