I think the guts of the problem is that many images to be displayed are not tagged properly, such that a proper implementation will display them "wrongly", and an improper implementation may have a better chance of displaying them rightly. Thus app makers (especially web browsers and other g.p. image displayers) are in a "danged if you do / danged if you don't" situation, and therefore opt for max likelihood of success (or less catastrophic failure) over a proper implementation according to current spec. Probably when more of the images to be displayed are created properly (versus those that aren't), we'll see implementations that follow suit. It's too much to ask a browser to try and be "correct" when too many of the images it has to display, aren't created/tagged correctly (and will be displayed wrongly).
Lr always does the right thing - because it's a photo processing app, not a general-purpose image display app. (and Photoshop has options for users who know what they're doing...)
It's not as simple as | icc-profile? - interpret according to it; no icc-profile => sRGB |. The spec allows for omitting icc profile in case of AdobeRGB, as long as *other* tags properly indicate correct handling - that's where the rub is.
Note: I'm not an uber-expert on this stuff, so take with salt, but I do know enough to believe the app programmer's gave this stuff some serious consideration before implementing as it's implemented. e.g. Firefox developers considered whether to make mode '2' or mode '1' the default, before deciding, no doubt reluctantly, to go with '2'.
PS - it's one thing if (time-consuming) development is defered (e.g. 30-bit display on Macs), and quite another when all the technology has already been developed, in which case it's just a matter of applying it. I'm not defending Apple's decision about this (I don't know enough to defend or criticize it), but again: if smart people seem (from the outside) to be doing dumb things, it's usually (read: not always - sometimes smart people can be dumb too 
) because the people on the outside who think it's dumb are not privvy to the same information that the insiders have 
(or they just don't understand the context in which various considerations have to play out...).
Another (potentially) interesting tidbit: Adobe's flash player has color management disabled by default, and the flash app used in Web module did not (does now?) display AdobeRGB images correctly. All you have to do is enable color management in a flash app and AdobeRGB images will be properly displayed on the web. Seems dumb to me (that Lr's web module would not always properly display images exported from Lightroom), but there you have it...
Cheers,
Rob