Only 20% of people who use bikesharing wear helmets!!!! OMG. OMG.
Of course the same study cited by Dr. Christopher Fischer - the one where he gets the claim that ""Head injury accounts for about a third of all bicycle injuries and about three-quarters of bicycle related deaths, so these are some pretty shocking numbers," - shows that normally 23.3% of people wear helmets. Another source I found put it at 25%. So it's more accurate to say "People who use bikesharing are pretty much normal."
Even if you don't believe that number, a statement about how many bike share users wear helmets is somewhat irrelevant without knowing the rate in the general public. They claim that less than half of cyclists don't wear helmets, but they don't give an actual number.
Nor do they follow up with any data showing that as a result there are more head injuries among CaBi users, which is what they're implying. Instead they rely, again, on the same flawed, discredited 1989 study that claims that helmet wearing decreases the risk of head and brain injury by 65 to 88 percent.
They do however ignore some relevant facts in their fearmongering. Their claim that 1/3 of all bike injuries being head injuries is a based on a study that showed that "83% of children aged 0-4 suffered head injury, compared to 42% of 5-9 year olds and 23% of 10-14 year olds." Among those over 20 it's closer to 1/6. So that number is thrown off by the presence of children - who aren't allowed on most bike sharing system.
That all being said, you're probably better off wearing a helmet.