Even in a nonauthoritarian country, defecting means you'll never return again to see your friends and family anyways. Given that, defectors probably consider making a political stand more important than their personal connections regardless of what happens after they leave. Not everybody has a loving family and close friends they can't give up.
You can't return but what is preventing them to come to your new home. Snowden's girlfriend moved to Russia and I think any of his family members can visit him if they want.
I meant that if he was willing to put his family through that much trouble, they probably weren't that close in the first place. In which case, what does or doesn't happen to family left behind isn't a large factor in deciding to defect.