They point out in the post that they cant justify the costs of using something like urban airship right now, based on the initial 'revenue' per sale. Theyre looking into maybe a subscription service:

    "Hopefully we can get the price per user down enough that it can just be a free
    feature for everyone. If not we may have to resort 
    to a nominal yearly subscription fee for the service."

Would you be willing to pay $2-5 a month? I imagine that something like Urban Airship isn't very cost effective if they offer it for free to everyone. On a side note, I don't understand why people just don't use Boxcar or http://push.ly/

As they mentioned in the post, Urban Airship costs money (after the first million messages per month). Also, they'd need to build the infrastructure to poll the Twitter API for updates and push them to Urban Airship--definitely not trivial.

Native push notifications aside, Boxcar (as the post says) does seem to do a pretty good job of picking up the slack while they figure out the best route for them.