I'm going to go out on a limb here and suggest that maybe MS is okay with Bing returning results, even pirate results that Google does not. Maybe MS agrees with the Pirate mantra that piracy doesn't hurt, but help sales. What would happen if pirates, even casual pirates learned that Bing results weren't filtered but Google's were? I would suspect a massive uptake in usage if this was proven. And who are these pirates…
Even if they lose some sales to piracy, as long as network effect work to their advantage, it's worth to lose it to a free version of their product than to a competitor.
It's unsurprising if they turn a blind eye to piracy when they judge it benefits them.
Having said that, this looks a lot like incompetence, or a contractor that's being paid to issue takedown notices only to Google.