That’s actually not true – it just depends how and why it’s done. According to halacha*, women are allowed to posken if they are qualified, but it’s doubtful if any woman nowadays is qualified, and most people who think they are, are not and are not doing it for the right reasons.
*source: Halichos Baisa by Rav (Dovid?) Auerbach, quoting Sefer HaChinuch (I’m pretty sure) and maybe others (don’t have it on me, so can’t look it up).