I remember reading about a recall regarding this topic when the F30 first came out. The headrests originally could be moved completely down, but this apparently violated some US safety spec for height (although they were totally correct in Europe).
BMW then made a minor change (and had a limited recall) so that the European headrests still go completely down as before, while the US headrests are fixed to stay slightly up -- unless you use the DIY workaround. I ran into this headrest problem with an F30 loaner car this week -- I also found the headrest to be too high for me (I'm not 6'-1", and yes, I know how to properly adjust a headrest).
BMW probably needs to find a more-expensive fix than just limiting headrest movement for US models.