Why are Boks back for Cup semis?

I am not a huge rugby fan, but I do enjoy watching and have followed with interest the Currie Cup competition this year.

The four teams that reached the semifinals did so largely without their Springboks, who were away on national duty.

Why then are those players now allowed to return for the penultimate round of the competition when it is other players who got their teams there?

I think this in unfair on those players who will now be left out, despite their earlier efforts. Confused