I found the list of objections presented by the FMI to NIST back in 2002:http://ts.nist.gov/WeightsAndMeasures/Metric/upload/fmicomments.pdf
Since the metric-only label option would be just that, an option, the FMI must feel that a lot of manufacturers would start to use that option, if it were made available to them. Furthermore, they must also feel that allowing metric only labels will ultimately lead to rational metric sizes.
