Why aren't the emoji modifiers GCB=Extend?

[Karl Williamson]

Someone writing code using Unicode 8 found that the FITZPATRICK 
modifiers are considered separate graphemes from what they modify.  This 
is surprising, and seems contrary to not only the concept of a grapheme 
cluster, but the spirit of tr51  2.2.3 "A supported emoji modifier 
sequence should be treated as a single grapheme cluster for editing 
purposes"