However, this time it's not catch - it's the full, entire version of Windows 8.1. The only limitation is that the manufacturer cannot change the default search engine from Bing to for example Google, but you as the consumer are free to do so if you like. And it's not very relevant for Digital Signage use anyhow :)
This is a serious cost-changer that easily shaves $100 or so from the player device cost - check out the excellent Zotac Pico 320 for example.
You can read more info about Windows 8.1 with Bing here.