Originally Posted by JohnAx
Would it be helpful to understanding this process (should anyone care but me) to ask "Why would BA do that?" Do they wish to encourage travel by certain ethnic groups over others?
You can certainly rule out encouraging travel by "ethnic groups". BA doesn't care who flies with them as long as they are paying the fare. If anything, it is to do with encouraging (or rather facilitating) competitive travel from areas outside the UK. For example, on the FRA-NYC route, BA competes head on with LH. BA offers a fare (DFFEUR) for £1493 (exc taxes). LH's nearest is (D1USW) £1944 (exc taxes). BA undercuts LH because it needs to incentivise travel via BA to offset the hassle (significant at the moment) of transiting through LHR. However, on the FRA-LHR (and v.v) sector, BA needs to make D class available for German travellers in order to realistically offer the fare to the German market. From the London end, BA wants to restrict the availability of D class tickets, because this is a prime business route with lots of opportunity to sell full J (or C) class seats. Hence the commercial need to have separate availability depending on whether it is a UK customer looking for a D class seat or a German customer looking for a D class seat.
Originally Posted by JohnAx
Or are there still SITI type rules that cause the price of a segment to vary greatly by purchase point (as is still very true for the rtw fares themselves.
SITI/SOTO was abolished some time ago. There are no remnants of it, as far as I am aware. Differences in fares originating from different destinations is a common occurrence; they are linked to the need to remain competitive in particular markets. This applies across the board, and not just to RTW fares.