Who says they'll sue their customers?
They might use this to protect their customers who face lawsuits from other companies.
Remember the Motorola vs. Apple and Microsoft FRAND lawsuits? Those were initiated the following way:
1) Motorola has blanket FRAND license with Qualcomm that covers Qualcomm's customers, requiring a payment of 2.4% of what Qualcomm charges those customers
2) Motorola discontinues that license specifically and only for Apple and Microsoft, obligating those companies to seek FRAND licenses directly with Motorola
3) Motorola demands 2.4% of the entire sales price of the products they sell using Qualcomm chips (in this case, iPhone and XBox, thus an increase of well over 1000%)
4) Apple and Microsoft refuse to license under those terms
5) Motorola sues
The courts haven't allowed these suits to get all that far, but they're faring better in Europe than they have in the US. These patents would allow Qualcomm to fight back against these sort of attacks on their customers.
I have no idea whether that's what they're going to do, but I think "suing their own customers" is a highly unlikely result of Qualcomm's patent purchase, because, as you say, its a stupid thing to do.