If someone has the ball and you kick the lower half of their leg you are not going to get sent off unless you stamp on them, use excessive force, go in two footed etc....
If they haven't got the ball then it's just a kick and you will get sent off. That's why Beckham got sent off, although it was more of a tap than a kick and could hardly be called violent so the red was harsh. It's not at all relevant to this discussion in any case.
The chap that Fer tripped had the ball. He wasn't lunging two-footed, he wasn't high, he didn't stamp, it wasn't with excessive force etc. It was a cynical trip. Yellow card
What you are saying is that all referees are applying the law incorrectly apart from this one trail-blazing hero.