<font face="Verdana, Arial, Helvetica, sans-serif" size="2">The problem is you have to fly in on a Star Alliance Carrier.</font>
The ASEAN air pass (actually 3-6 Y coupons @ $120 each) is sold by the national carrier of every ASEAN country, in conjunction with a longhaul on one of those carriers (not Star Alliance).
Participating carriers are BI/GA/MH/MI/PR/TG/SQ/VN/QV/8M.
Practically speaking, that often means earning UA miles on SQ and TG, as some of those airlines don't have foreign partners.