Why they don't implement nofollow attribute in ads, have you check juicyads ads, you will see they are using nofollow attribute in ads, so Google pagerank is not pass to that URL, Also Google highly recommodend to use Nofollow tag in all advertising links but look's like plugrush technical team don't know anything about it.
If plugrush can't use nofollow tag like other adult ad network using then they can use robots.txt method as well.
Look out Google official Ad server i.e. Adsense.
Code:
Googleadservices.com/robots.txt
As you can see they block most of bots including Googlebot, MSNbot etc, so pagerank will not going to pass, so that is other method for ADS SEO.
I really like if Plugrush implement that kind of thing, If any of plugrush team hear me then please implement this, and if you want to know more about this, then feel free to continue this discussion here.