IANAL, but I believe that particular practice is prohibited under the auspices of the Universal Declaration of Human Rights, which, while not legaly binding as such, forms the linguistic basis of legal constructs within binding covenants and treaties to which the United States is a signatory.
The fact that this practice continues should surely be pursued with the utmost priority by all those concerned with probity and justice among civilized peoples.
Are you joking? Maybe this is just sarcasm bordering on being a little dark, but it would be funny. This is where seeing someone's face uttering the words would convey a lot more information.
http://en.wikipedia.org/wiki/Cafe_Hon#.22Hon.22_as_a_tradema...
It also seems like a lot of amateur t-shirt sites have tons of custom i <3 shirts.
On a possibly related note, here's something I've been wondering about. A few years ago phrases like "I love LA" or "I love New York" became "I heart <whatever>". Is this also some kind of a copyright issue? It sounds so ridiculous that I couldn't imagine radio and tv stations using using it if not by force.