The article is wrong on this point: the fact that he was crossing state lines was not a pretext for the stop.

They actually wanted to add another charge. The cameras helped them find the right opportunity to do that.

If they just picked him up driving around, they would only be able to charge those existing crimes.

By waiting for him to cross state lines, they have a chance to add on another charge.

They already had enough PC for the stop, and they could have searched the car as incident to the arrest at any time.