That's like advertising a new car with excellent mileage being one of the key features.
and during the first demo (or preview if you will)? Ran out of the fuel exiting the garage.
Do I expect it to be perfect in b1? Nope.
Should it go completely against one of its key features? Not really.
Should we understand that the average dev's testing phone is his daily driver? Kinda.
Beta software is not a demo nor is it a preview. A demo is meant to demonstrate something to the public, while beta software is meant to test software so that bugs can be eliminated, and so that final improvements can be made. It is not made for public mainstream consumption. Even public beta testing is generally limited and not meant for mainstream consumption. Betas often don't come with everything that has been advertised for a final version.
Apple was probably fully aware of the bad battery life when the first beta was released. But they likely don't care at this point... And are focusing on other things... That in the end will make it all come together. Apple doesn't focus on making beta versions of software stable enough for people to be reliant upon to use as a daily driver... So if you think that Apple owes you anything during the beta cycle you're highly mistaken.
The developer preview/beta program is not intended for you to have a stable version of iOS 9 earlier than everybody else. It's meant to help app developers test apps on its new operating system and have the opportunity to implement new APIs. And it also helps Apple to work out bugs... Public beta testing helps this on a much grander scale... It will likely have more stability but you still shouldn't bring yourself to expect anything.
The point being if you want to run beta versions of software and have access to it earlier than everyone else... Great!! But expect bugs, stop bitching about it, and don't act like apple owes you anything... Because, after all, it's a Beta.