This is somewhat true. I'd imagine IPS factors three things into whether they'd take action on a nulled community:
[*]Size - is the community large enough to earn a profit, that it's unacceptable that they haven't purchased a theme?
[*]Content - if the community is already in the business of distributing pirated content or sharing other questionable content, they'll likely pursue legal recourse.
[*]Worthy - really a combination of the two points above. IPS will try to determine if it'd be worth their time and resources to take any legal action against a community using a nulled board.
Essentially, if you're a smaller community just trying to get by (even with a sizable member count), IPS will likely ignore you.