Aiming at the cash
This is not entirely surprising. The patent trolls will naturally go for the wealthier companies, and they don't get much wealthier than Apple. The richer they are the less significant the consequences of just paying up. There's no point suing a close-to-bankrupt company, there'd be no profit in it.
Of course, Apple's own litigious tendencies will probably mean that there won't be much sympathy...