Is it legal for a US citizen to put US passport inside an EU nation passport cover? When I say "legal" I mean when the person forgets to take off the cover when standing at the CBP counter at port of entry.

Example:
http://www.ebay.com/itm/Italian-Italy-Genuine-Leather-Passport-Cover-Holder-Protector-Europe-Union-New/331799552552?_trksid=p2047675.c100623.m-1&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D2%26asc%3D40130%26meid%3Dcb53c3dc64df461bb5e0ece3b7b64fa4%26pid%3D100623%26rk%3D3%26rkt%3D5%26sd%3D122313561116