It can now be found in the CLI under
[admin@MikroTik] /interface ethernet poe settings> print version: 2.13 ether1-poe-in-long-cable: no
Instances where you might use this is if your device that you are powering requires more than the 500mA than is being provided on POE-Out ports.
Be advised that this is dangerous and you should only use this option if you know what your doing.
For further reading: http://wiki.mikrotik.com/wiki/Manual:PoE-Out