Many of the large 135 operators require one. I don't see the problem, as long as it's reasonable and somewhat reflects the training cost the company invests in you.
Our shop is great about it - it's pro-rated, it's somewhat reasonable, and it only kicks in after you've been with the company for a while (and it comes with almost 100% salary increase) so you know if you and the company are a good match.

The 24mo Caravan SIC contracts for $15k are a ripoff.