That is an excellent point, but..
It is correct info as far as what the agencies espouse, but a deco dive is a dive in an overhead environment. Lots of agencies make mistakes, and have inconsistencies for historical reasons. An agency that allows overhead environment diving without redundancy is wrong and PADI fits nicely in this group but for different reasons, as talked about in this thread:
http://www.scubaboard.com/forums/basic-scuba-discussions/486683-wreck-penetration-queuing.html
The degree to which an agency epsouses diving in overhead environment without full redundancy and advanced training is the degree to which that agency is trying to serve too many masters (usually members from way back).
They are also often agencies which still base their repetitive dive tables on the old US Navy deco tables or the equivalent which is the reason they have to address deco diving at all. Without, unfortunately, actually ensuring that the divers they train were the sort of exceptional physically fit specimens that the Navy Divers were, or ensuring that their divers were fully supported with surface deco chambers, on site medical staff, etc.
PADI also mistakenly allows overhead environment diving on the recreational level, for different historical reasons: they offered these Cavern and Wreck classes as a money grab before they decided to go ahead and actually have a technical training side, and they don't want to revoke training credentials of members from way back.
Again, breaking/losing a mask happens often in diving. It is irresponsible to teach deco diving/overhead environment diving without requiring redundancy because divers without masks cannot do even see their SPG let alone their computers/tables. Only allowing direct access to the surface makes it safe to dive without a backup mask.
It's irresponsible to dive in, or allow diving in, overhead environments (wreck, cave, deco) without redundancy and advanced training, or experience, when the blowback can land on others heads. Many people do the diving, many instructors train in overhead environments, and as you point out there are even some agencies who are wrong on this issue, PADI among them.
The biggest issue here for me really is, diving off a boat. If someone wants to do solo non-rendundant deco dives from shore, then that is kind of their own deal. But diving on a boat owned by a company, with a captain, is necessarily something different. Most insurance coverage has specific exclusions/inclusions for various activities.