Businessweek has an article that saying that 95% of ATMs worldwide are still using XP. Support for embedded XP ends in 2016, instead of this year for regular XP. It wouldn't be surprising if POS systems have similar ratios.
Embedded operating systems often have customized kernels to accommodate the lower-end hardware in most POS and ATM devices. That being said, it’s still a Windows XP kernel which is a well-known vector and capable of exploitation if not patched properly, which I can personally attest they usually aren’t.