The Tau are smart enough to see a warp rift, see the murderous horrors pouring from it and say: "We should probably shoot them." Daemons wouldn't put on an act of civility and friendliness as they're vomited forth from a warp rift, and they certainly couldn't feign diplomacy like the Haemonculi did. The Tau would recognise them as a threat and fight. The only reason they get allies is because either they're summoned by worshippers or they've been whispering into the minds of mortals. The Tau are not susceptible to this.