A card for a high tackle.

When it comes to foul play, the action a referee takes is at his own discretion. There is no hard-and-fast rule. He may consult his assistant but in the end the decision is his.

Not every high tackle means a yellow card. Not every air tackle equals a yellow card. Not every punch requires a yellow card - and so on.

If there is foul play, the referee is required to act. In the case of a high tackle, the referee must penalise.

Law 10.4 (e) Dangerous tackling. A player must not tackle (or try to tackle) an opponent above the line of the shoulders even if the tackle starts below the line of the shoulders. A tackle around the opponent’s neck or head is dangerous play.
Sanction: Penalty kick

Even trying to tackle high is seen as an offence.

Law 10.5 SANCTIONS
(a) Any player who infringes any part of the Foul Play Law must be admonished, or cautioned and temporarily suspended for a period of ten minutes’ playing time, or sent-off.
(b) A player who has been cautioned and temporarily suspended who then commits a second cautionable offence within the Foul Play Law must be sent-off.

Any part. That includes playing "against the letter and spirit" of the Law, including deliberate infringement. The referee must act against it and he has three ways of dealing with it:

a. a ticking off - admonishment;
b. caution and yellow card
c. a red card.

Which he chooses is up to him and will depend on how grave he believes the offence is.

In this case the referee considered Gerhard van den Heever's high tackle on Frikkie Walsh serious enough to warrant a yellow card.