The web+ prefix is meant to designate that a website has registered that protocol handler; Bitcoin isn't a website. The origin of this limitation is probably security concerns, for instance registering protocol handlers for protocols that have no reason at all to go to a website. Note how chrome has a few exceptions like mailto: for where it is agreed that websites should be able to register protocol handlers. Chrome can eventually add bitcoin: to this list if the chrome developers think it is a good idea. In addition it looks like Firefox does not have this limitation.
Hmm, I read the Mozilla page and saw "web+" mentioned there as a security constraint, so I thought the protocol aims for this to be a global restriction. In fact, it's just listed as an implementation detail in Chrome, so it's too specific to consider.
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