From the PBOT Bike Share FAQ Page: 

"What about helmets for bike share users?"
"Portland has a culture of dedicated helmet use, as over 80% of cyclists wear helmets in the city -and the percentage continues to grow. Portland bike share plans to capitalize on that behavior by encouraging helmet use through free helmet promotions, discounts for annual members to purchase helmets, and low-cost helmets for low-income users through retail partnerships. The City is also exploring helmet vending options in conjunction with bike share station locations and other high-visibility areas."

First off, I find the 80% helmet use statistic incredulous. By my tally, helmet use in Portland is more like 50% and getting smaller. BikeShare should never have started without a helmet in every single basket, free of charge. I was hit by a car on my bike, hit the pavement, and my helmet broke into several pieces. A year later I still can't count to 100 without making a mistake. Concussion is life-changing.