Indeed support for 802.3af/at seems like a small magic in Routerboard devices. Even official compatibility matrix
shows need for crossed cables with some RB devices, including cAP lite.
The table actually suggests (based on columns 2 and 3) that some RB devices are picky about polarity (and not all of them prefer same polarity) while some devices aren't.
PoE article on wikipedia
offers some explanation about need for cross-over cables (in the Powering devices
section). Seems like PD (power sink) is responsible to implement kind of a "auto MDI/MDI-X" operation for PoE, PSE (power source) will always output same polarity.