Well there are two things you need to verify and it sounds like you're pretty certain about the first thing. Are you certain this line was run across your property and outside of an an easement or the right of way? Secondly, you should verify that by agreeing to allow Comcast to provide you service that you have not given them permission to extend their service to additional customers across your property. Secondly, is it possible that this is a temporary line because there's no reason that the line should be exposed. This just doesn't sound like anything a reputable cable company or contractor for a reputable cable company would do.