If the UK chose not to allow dual UK/Scottish citizenship, that would make Scotland the only country that the UK wouldn't allow to have dual citizenship with the UK.
That's not what they are saying. You are confusing dual citizenship with keeping UK citizenship after independence from the UK. As I explained above, some Scots would be dual UK/Scot, as they will qualify for a UK passport in their own right, just as as citizens of all countries can do i.e by descent, residence. The UK does not stop dual citizenship.
But the precedent for all the countries that become independent of the UK, is for the citizens of that new independent country to lose UK citizenship and to take up citizenship of their own new country instead. That is what being independant means after all, forming your own new country, with your own passports; your own currency and not being part of the UK.
Who would gain from that? Nobody has said that Scots born after independence would automatically have UK citizenship, just that people who are ALREADY UK citizens would not be stripped of citizenship the day of independence.
That is not what your scot refendum states, as I linked above. They clearly tell you that "
It will be for the rest of the UK to decide whether it allows dual UK/Scottish citizenship".
Badly worded by them, as of course there will be some Scots who can be dual UK/Scot if they qualify in their own right, descent, etc, but what Scot Referendum means is that the rest of the UK will decide whether to let the Scots
keep UK citizenship when they decide to leave the UK. Precedent has always been not to let independent countries citizens keep their British citizenship, when they leave. Why would you expect to keep citizenship of the UK if you have voted to leave the UK?
The situations you are talking about with past colonies involved the fine distinctions between being a national, a citizen, and a subject; it's not relevant to current British nationality law, and not relevant to full British citizens, which Scots are, versus British subjects, which the former colonies were.
Scots became British when they asked to join the union. If they leave the union, they will then cease to be British, unless they qualify in their own right for UK citizenship.
As I said above, even if Scotland does offer the UK a good deal so that the UK does decide to break with precedent and let Scots keep UK citizenship at independence, UK citizenship will start to end for Scots within 20 years. Their children born outside the UK will be British by descent and their children born outside the UK, will not have UK citizenship.