Yes, it's by design, as the request for campaign events is intended to provide links or the event details, or details about contacts, dates/times for each of the detected events. We do not reveal which contacts submit SPAM complaints, so there are no details to protect the privacy of those who file them (for CAN-SPAM Act compliance). To get just the number of SPAM comlaints at this time, you can make a request on the Campaign Resource, and the number of SPAM complaits will be included in the campaign details returned.
I hope this is helpful to you. Let us know if you have any other questions.