The following xml syntax normally pulls back an ASIN if the release has a relationship to one. These 7 examples don't.

Web page (with ASIN): http://musicbrainz.org/release/f4e89a73-6251-4e52-b188-08383eda6ccc
XML (without ASIN): http://www.musicbrainz.org/ws/2/release/f4e89a73-6251-4e52-b188-08383eda6ccc?inc=recordings+artist-credits

Web page (with ASIN): http://musicbrainz.org/release/719826bd-2259-4cc0-9a2e-bbedeaf5007d
XML (without ASIN): http://www.musicbrainz.org/ws/2/release/719826bd-2259-4cc0-9a2e-bbedeaf5007d?inc=recordings+artist-credits

Web page (with ASIN): http://musicbrainz.org/release/50398938-ce77-40b4-8106-5ea18365464c
XML (without ASIN): http://www.musicbrainz.org/ws/2/release/50398938-ce77-40b4-8106-5ea18365464c?inc=recordings+artist-credits

Web page (with ASIN): http://musicbrainz.org/release/cac488fe-b4e0-45f9-8879-2ad1d4d507b2
XML (without ASIN): http://www.musicbrainz.org/ws/2/release/cac488fe-b4e0-45f9-8879-2ad1d4d507b2?inc=recordings+artist-credits

Web page (with ASIN): http://musicbrainz.org/release/ca211805-0ba9-43c3-8ca0-7c7ca8ecacc0
XML (without ASIN): http://www.musicbrainz.org/ws/2/release/ca211805-0ba9-43c3-8ca0-7c7ca8ecacc0?inc=recordings+artist-credits

Web page (with ASIN): http://musicbrainz.org/release/afc73e89-68cf-47a1-84a6-d736c58c48a4
XML (without ASIN): http://www.musicbrainz.org/ws/2/release/afc73e89-68cf-47a1-84a6-d736c58c48a4?inc=recordings+artist-credits

Web page (with ASIN): http://musicbrainz.org/release/5bc052b2-5ef3-4b0c-a590-71b865bbccd7
XML (without ASIN): http://www.musicbrainz.org/ws/2/release/5bc052b2-5ef3-4b0c-a590-71b865bbccd7?inc=recordings+artist-credits

Cheers
Daniel