McCloskey,
Who said anything about a rule? The 2 examples you bring up were extreme circumstances, and I don't think anyone (USGA, players, or audience) was all that happy with the way those 2 greens played. As a spectator, watching those 2 tournaments, I wasn't riveted by 1 green out of the whole course being slower.
Most of these old courses have been taken past their speed limit, and then you are faced with 2 options. One is to slow all the greens down to match the one or two that are unplayable, or alter the greens in question. Personally, I am not in favor of alterations to accomadate speed, but it has been done.
A course that I worked at had a green which we mowed higher, and it was still unplayable. This fall the green was altered.
IMHO, I think mis-matched speeds is goofy. There are other alternatives to bringing down scoring other than what you suggest.
Doug