Well the coles notes is that the ACO decided that they were going to put controls in to ensure that the GT ranks did not produce 'prototypes'. The ACO felt that 'GT' was getting beyond what it should be, and implemented rules to stop this.
There are those who will suggest that Porsche complained, and while they surely did, I think that many give Porsche more credit than they deserve in changing the ACO policy.
While it is disappointing to lose a quality entrant such as BMW, going forward I think that these rules give more stability to these classes.
Basically, BMW would have had to produce 100 cars to receive no penalties. The cars have to be as with original block, in original location, and orientation. The BMW M3 GTR had a V8, and there has been no production road cars of this type. I understand that the car was originally allowed to race under a verbal promise that there would be production cars within the year. As of yet, no cars produced.
By the way IMSA/ALMS run under ACO (LeMans) rules. The rules for LeMans and the ALMS are the same.