Another thought:
The laser pickup in an optical drive really uses three beams. It splits off some light to the left and right to keep it aligned on the track. The drilled spots are like little concave lenses and would refract the side beams so they lose their tracking signal. To the drive electronics it would appear that the disc suddenly disappeared.
I guess the reason for this is that if a drive can't make an image of the disc (because of the above failure), it won't be possible to burn a copy. Also note the holes are quite close to the center (near the beginning of the track). You would hit them only ~20 MB from the start. If the pressed master contains a table of contents that is (e.g.) 650MB, then a copy will always be distinguishable because its data will end too soon, so its table of contents will also be shorter.
