Actually, they didn't. Sun Tzu had a chance to nominate a single faction to be vetted to join the second Star League, and chose to nominate the Taurians. And even they weren't formally inducted until 3067 in order to attempt to stave off the collapse of the second Star League.
They did ally themselves with one of the factions in the Star League, and were oddly part of several actions of the Star League (Danai dying on Huntress, partially due to Victor's incompetence, and Naomi being on the panel that judged Kali Liao for the Black May attacks), but never joined, or were even vetted to join.
Though to be fair, even when the first Star League attacked, they weren't the only ones fighting back :)