Just thought I would put my 2 cents in. I own private trees and our DNA is pending at the moment. Have already DNA tested with other companies. There are a few reasons I keep my tree private for one I have spent thousands of dollars on court records not found on ancestry. Most of the people I research with we share the cost of court records but some people want everything for free. Like someone posted in here the cut and pasters!!!
Second, some people testing like my family have adoptions. Some people who get DNA matches and find out there are "Oops" in their family don't take kindly or accept that their father, grandfather, mother, grandmother ect had an unkown affair. Most adoptees who test have no family tree and therefor cant provide you with any information. Most adoptions are sealed in a court of law in most states. Adoptees getting 5th cousin matches wont help them. They need close family member DNA matches to figure out who they are related to.
As for other people who are not adopted and may have no tree, they may not have an ancestry account and why they have no tree. You can DNA test and not be a member of Ancestry. Some people who tested may have also discontinued their ancestry account and why you may see a DNA match without a tree.
If any of you chose to ignore private or no tree issue it may be your loss as well as the person who tested.