I look at it this way. I pay for my phone service. I only give my number out to those I want to call me. Telemarketers are freeloaders.
My wife and I put our names on the do-not-call list for telemarketers. We feel strongly that since we pay a monthly fee for telephone service to our home that these marketers are freeloading, using our lines to carry on their business.

This is just the same as going into someone's private office and freely using it and its equipment to carry on your own business.