Because if applicable to the idea of morality then it would fail for the same in coherency. You are effectively making jakswan's point for him.
Not entirely sure what you mean.
Position A:
"1) All men are human
2) David Cameron is a man
3) Therefore David Cameron is human
1&2 logically lead to 3. And subscribers to 1&2 are logically bound to accept 3."
Position A will not be troubled by substituting something incoherent into it such as:
Position B:
"1) All men are human
2) My dog Rex is a man
3) Therefore my dog Rex is human
1&2 logically lead to 3. And subscribers to 1&2 are logically bound to accept 3."
Great, this demonstrates that if you start with absurd you end with absurd. This does nothing to rebut the original position.
In Alan's case you could claim that the notions of morality being independent of minds and human agreement, and that a moral value would still hold even if everyone who believed this died out, are incoherent or false, but this will need a separate argument.
You will achieve nothing to trouble it by substituting something that probably is incoherent in its place. As the notion of an experiential concept such as how things taste being independent of minds and senses, is.
Once again, Alan's notions and argument may or may not be incoherent or false, but the specific tactic employed by Jakeswan of applying the same position to taste does nothing here.