Why? There's a possible oil pump issue that can result in a sudden loss of oil pressure and severe damage to the twin-turbo V8. No risk of fiery death... yet.
The stop-sell is only applicable to model year 2013 M5s and M6s. A bulletin was just issued (obtained by Bimmerpost) that places a hold on deliveries of the cars and recommends that current owners not drive their cars.
BMW is apparently reaching out to customers that have already taken delivery of the cars to advise of them of the issue. If a sudden loss of oil pressure occurs, BMW has recommended that owners stop driving the cars immediately and have them transported to a dealer on a flatbed.
Dealers don't seem to be ordering parts to fix it yet, but that just might change soon.