The rules don't actually require the feet to be set, only for the defender to have established his position in front of the offensive player. The feet are commonly considered a marker to whether the defender has established position, but that isn't necessarily the case.
The rule as written is basically that if the offensive player tries to go through the defender, it is an offensive foul. If the defender pops up suddenly without giving the offensive player the capability to stop or change direction, then it is a defensive foul.
It's really one of the worst officiated rules there is.
The other being a the whole block called in the no-charge zone. According to the rules, it is not an automatic blocking foul in the restricted area, it just cannot be ruled as a charge if a secondary defender establish position in the restricted area against anyone other than a player who received the ball in the general vicinity of the basket. It is however always called a block, which shows that the NBA does not follow its own rules.