Generally, copyright protection is to ensure an artists's/cartonists's/author's work is not used by someone else for monetary gain without permission or compensation. Although I can see where it's technically illegal (like jaywalking) once a piece is on the internet, I'm sure the originator is aware his/her work is going to be shared.

For an environmental conference I was organizing I once contacted Larson's office to ask if we could reproduce a cartoon as part of the advertising for the conference. I explained we were a not-for-profit organization (NH Dept. of Environmental Services), we would not use the cartoon to make money and it would benefit the environment. Larson's staff was sympathetic and said they understood his work was commonly reproduced for a variety of reasons but preferrd we wouldn't use his work as a general policy. They stopped short of saying they'd take legal action if we did. We ended up honoring their request.