What does it mean that the "The Evil One snatches away what was sown in them." I have always been confused about what this means.


  Those who hear the message about the Kingdom but do not understand it are like the seeds that fell along the path.  The Evil One comes and snatches away what was sown in them.
  - Matthew 13:19


What does it mean that the Evil One snatches away what was sown in them?  It sounds like a violation of free will to me.
The defining characteristic about the path is that it is hard, like a sidewalk or road. People represented by the path are hard-hearted. Their hearts are not receptive to the word. They are the people described by Paul in 1 Corinthians 1-3.


  1 Corinthians 2:14 (NASB)
  But a natural man does not accept the things of the Spirit of God, for they are foolishness to him; and he cannot understand them, because they are spiritually appraised.


What Satan "takes away" need not be their choice in the matter but the chance to think about the message. Among his tactics of distraction are pleasure, busyness, etc. His "taking" is also not against their will, but rather their acceptance of his leading. He has no real power over our choices except what we give him.

Matthew Henry refers to these people:


  Careless, trifling hearers, are an easy prey to Satan; who, as he is the great murderer of souls, so he is the great thief of sermons, and will be sure to rob us of the word, if we take not care to keep it.


Adam Clarke describes them as:


  A careless inattentive hearer is compared to the way side-his heart is an open road, where evil affections, and foolish and hurtful desires, continually pass and repass, without either notice or restraint. "A heart where Satan has" (as one terms it) "ingress, egress, regress, and progress: in a word, the devil's thoroughfare."