And you know why? Because the vast majority of shippers don't see slash on the show.
I don't have anything scientific to back that up; it's just a personal observation, and one I feel quite confident about. And of course that's not to say that there aren't shippers who understand where slashers are coming from. Doesn't mean that shippers are anti-slash, either. Just means that that line typically isn't crossed. Yet some slashers seem to assume that Sam/Jack fans live in constant fear of Jack picking Dannyboy over Sam.
The truth is, most of them - at least the ones on my list - like Daniel just fine. He may not be their favorite character, but I wouldn't expect that any more than I would expect Sam to be a Jack/Daniel or Jack/Teal'c or Teal'c/Daniel slashers' fave. Some of them, especially in recent months, have been less the impressed with 'Danny's actor', but again that's another matter.
Many, many Sam/Jack fics which include Daniel put him in a matchmaker role... cliched, perhaps, but hardly negative. Ship writers often cast him as the person that Sam and/or Jack can talk to, confide to about their feelings for the other. Never have I seen a story that casts Daniel as the evil temptress trying to steal Jack away from his beloved. He's in no way portrayed as the enemy, and his character is not degredated.
On the other hand, what I've read in the few slash fics I've come across... well, I won't go there, for the plain reason I have read so few. I've no reason to believe that slash fics that show Sam as an evil bitch trying to lure Daniel's man away are the rule and not the exception.
I think it comes down to a couple of things. I think some folks are probably so flabbergasted by the idea that someone doesn't adore Daniel that they have to come up with ridiculous reasons as to the 'why'. The perception that one is threatened by the character is as good as any. And I think some JDers are projecting their own dislike of Sam (for breaking up the Dynamic 1-3 Duo of Colonel Jack and Doctor Danny - heh) onto their 'polar opposite', so to speak.
And some probably have oatmeal for brains and couldn't think their way out of a bad Enterprise plot. Alas.