The BOM is optional in UTF-16 too:

  The UTF-16 encoding scheme may or may not begin with a BOM.
  However, when there is no BOM, and in the absence of a 
  higher-level protocol, the byte order of the UTF-16 
  encoding scheme is big-endian.

(D98 in http://www.unicode.org/versions/Unicode6.1.0/ch03.pdf)