Not everybody needs AC3 5.1 sound. If Adobe would offer that as an inherent part of PR, the price would increase, due to the license cost and people who don't need it would pay for something they don't want.
That would be a valid argument if all the other NLEs didn't offer it, but they do, and except for the hugely overpriced Avid, they are all less expensive than Premiere.
You'd have to talk to the other companies to find out how they do it. Adobe has stated that they'd be required to pay a license fee for every copy of the DD 5.1 encoder, and that's why it's not included by default.
In the case of Sony Vegas, Sony has a direct licensing agreement with Dolby Labs. Because of the nature of the Sony license, they can offer DD 5.1 SS on some pretty inexpensive programs. Though I doubt that Sony would respond, I do agree with Jim Simon, that one would have to inquire of those companies, exactly how they do the licensing.
Adobe does not, so licenses the DD through Minnetonka Audio, and the SurCode encoding module.
Some NLE's offer 5.1 SS encoding, but not DD certified 5.1 SS. In those cases, the DD 5.1 SS logo cannot be applied to the DVD's, or BD's, as it can if the SurCode module is used, with but a little paperwork. I do not have a list of the certified, and non-certified DD encoding, but a bit of research on the Dolby Labs Web site might yield such info.
Also, Minnetonka does not charge that much more for the SurCode encoding module, above the licensing fee that they pay to Dolby Labs. I was surprised at how expensive that license was, relative to the price of the SurCode module, when I inquired about "volume licenses" from Minnetonka. They were very forthcoming with details, some years ago - also, at that time, they offered a special Adobe-user license deal, but that was dropped a bit later, and, unless something has changed, the fee for Premiere users is the same as buying directly from Minnetonka Audio.
Just some observations,
I do not have a list of the certified, and non-certified DD encoding
The issue with 5.1 isn't certification. Dolby Labs owns the encoding technology. There are no legal "non-certified" encoders. You either pay Dolby for the license, directly or through a third party, or you're using illegal software.