I'm probably more annoyed that the Kickstarter video basically has them begging for money just so they can get the Stern pinball app started with AC/DC like they can't fund a new game engine without Kickstarter whales. Oh well, that's capitalism. If they can get someone else to pay for it then why would they pay for it themselves?
The separate app is for Stern branding to help with their advertising that Stern probably wanted. They also say it's because modern pinball games are a lot more complicated to emulate so they need an entirely new code base to deal with that.