Unfortunately, life's not quite as simple as that.
Firstly, though there are standards for delivering a 7 day EPG, they're not perfect, and in any event you couldn't deliver both those and the data used by Sky on the same transponders at the same time. So a method had to be developed that would work alongside Sky, otherwise all the channels would need two transponders to be on each.
So, since that's being done, the decision was taken to make sure it's an advanced EPG and platform, going well beyond what's possible with the basic standards, with things like support for meta data to provide recording facilities equivalent to Freeview Playback.
And, to provide a service at least equivalent to Freeview (remember one reason for this is to give service to those who will never get DTT coverage) an interactive element was specified, using MHEG (same as Freeview), plus extensions that will ensure the equipment has a reasonably long life, including the possibility of IPTV and broadband return path in future.
And, having created those specs, Freesat want to ensure that any box with the logo meets them. The standard isn't a secret, but you do have to agree to the way it's implemented to sign up.