You're absolutely right, Shed. But if you look at the history of airport development over the past 40 years you can see a gradual shift between what is funded by public authorities and what is funded by airports.
Historically it was generally accepted that all surface access costs were publicly funded, eg the original Piccadiily Line to Heathrow in (?) 1977. But when the line was extended to T4 LUL demanded (and got) a contribution from BAA. By the time T5 came along BAA/HAL had to fund the lot.
The boundary between public funding and airport funding has been consistently moving in the same direction. Airports are now expected to fund a lot of projects that were previously publicly funded. The debate over R3 surface access is just a continuation of this.
What's sauce for the goose is sauce for the gander. If HAL eventually has to cough up for a range of surface projects that are only partially attributable to R3 then it's inevitable that all other future airport projects will also be regarded as a funding opportunity. A case can certainly be made that growth at MAN (for example) contributes to congestion on the M56, M60 etc. From there it's only a matter of time before MAG is asked for a contribution to costs. And that is different from MAG currently having to make a contribution to projects (eg Metrolink) which are much more directly related to the airport.