This is unprofessional and makes the whole industry look like selfish children. If you didn't get paid it is because you didn't handle the business end of your job correctly and get the proper contracts in place.
[EDIT, what follows was added after I originally posted]
Some people have pointed out that contracts don't guarantee payment, and that is true. What contracts can do is let the client know that they have to pay $X to start a project, $X at each stage, and then the final files are released to them after the final payment is made.
This is unprofessional and makes the whole industry look like selfish children. If you didn't get paid it is because you didn't handle the business end of your job correctly and get the proper contracts in place.
I highly recommend reading this book, twice: http://www.abookapart.com/products/design-is-a-job
[EDIT, what follows was added after I originally posted]
Some people have pointed out that contracts don't guarantee payment, and that is true. What contracts can do is let the client know that they have to pay $X to start a project, $X at each stage, and then the final files are released to them after the final payment is made.