James Harden popularized the double stepback.
Sometimes, it's called a travel.
Sometimes, it's not.
How is the double stepback legal?
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The double stepback is legal when used with a zero step, also referred to as a gather step.
More details from the NBA below, but a player may take two steps after gathering the dribble. Depending on when the ref determines a gather has taken place, it may or may not be a travel.
For the sake of the question, let's assume Steph Curry's first double stepback attempt was correctly called as a travel. This would mean three steps took place after the gather step had been determined (as annotated below).
The NBA defines a "gather" as:
For a player who receives a pass or gains possession of a loose ball, the gather is defined as the point where the player gains enough control of the ball to hold it, change hands, pass, shoot, or cradle it against his body.
For a player who is in control of the ball while dribbling, the gather is defined as the point where a player does any one of the following:
- Puts two hands on the ball, or otherwise permits the ball to come to rest, while he is in control of it;
- Puts a hand under the ball and brings it to a pause; or
- Otherwise gains enough control of the ball to hold it, change hands, pass, shoot, or cradle it against his body.
This is relevant as the following is incorporated into NBA traveling rules like so:
The gather will be expressly incorporated into the traveling rule to clarify how many steps a player may take after he receives the ball while progressing or completes his dribble:
A player who gathers the ball while progressing may (a) take two steps in coming to a stop, passing or shooting the ball...
A player who gathers the ball while dribbling may take two steps in coming to a stop, passing or shooting the ball.
In the end it's all up to the referee watching the play. Many illegal actions are uncalled in basketball as the referee misses them or chooses not to call them.
NBA rules state the zero step as:
The zero step is the first movement a player makes with their foot on the ground while receiving or gaining control of the basketball. This rule allows players to make their first and second steps after receiving the ball, before finishing their play. The zero step only applies to players that are moving while receiving the ball.
Thus, James Harden commonly uses this rule to his advantage.