I was being a bit sloppy when I wrote "encrusting corals". I specifically remembered a relatively recent thread where a tiger cowrie had chowed down on a non-softie coral. It turned out to be a plate coral:
http://www.canreef.com/vbulletin/showthread.php?t=40449
I also recall reading on Bob Fenner's site about a naughty cowrie. This time the victim was a donut coral (scolymia):
http://www.wetwebmedia.com/snailcompfaqs.htm
If you think about it, there's no reason why a tiger cowrie going along would differentiate between zoas, softies or any other fleshy matter (like the polyps of LPS). I wouldn't be surprised if given the chance a tiger cowrie would eat SPS including monti caps. It's just that most people keep their SPS higher up on rockwork where the cowrie can't get access.
It seems that as they grow, tiger cowries can become more omnivorous and become a risk to any type of coral at on near the sandbed.