You opt in for this, and it's kind of the whole point of the PSAT. You're taking the test so that colleges will send you discounted offers. There isn't much other reason to take the PSAT.
It's the PSAT/NMSQT, which means it's also the National Merit Scholarship Qualifying Test, which is an excellent reason to take it besides increasing your spam burden.
95% of test takers know before they start that they aren't in the running for the NMS.