Agree you might be cutting too low if you hit them but maybe your mower has really good vacuum action. Disagree respectfully that they won't cause blade problems. Walnut shells are used in abrasion blasting so obviously they are pretty hard. Pick em up. Great in cookies and healthy too
Time is money so you may or may not want to charge for picking them up. I had a customer who had a lot of pear trees. The fruit definitely cause minor complications because there were a lot of them on the ground. Cause uneven mowing at times. I suggested to the owner that he run an ad on CL for free pears. Worked out pretty well.
Do whats best for you.