These are enterprise grade routers and the b protocol is disabled intentionally. The protocols do not have to be backwards compatible. Most routers do support the mixed mode but leaving mixed mode on has downsides to performance when a b device actually connects (it often downgrades everyone else to b as well). Mixed mode was around before n (my old SMC g router could be "b only", "g only" or "mixed"). Our network engineers have chosen to disable support for b.
I guess my point is that if you say your device supports 802.11b/g/n that implies that it can run in all three of those modes. 802.11b/g/n itself is not a standard. It represents three separate standards. Marketing has shortened it by saying 802.11b/g/n when a device supports all three standards.