I don't understand why people are find this twist unoriginal. Am I missing something?
On this board, unoriginal/uncreative/bad writing is usually code for "they did something that wasn't 100% happy and I don't like it".
Your salt aside, it's more that it's standard for Kamen Riders' powers to be created by the bad guys, and "the hero was actually playing into the bad guy's hand all along" really is a fairly standard twist.