No doubt, all of the Seinfeld episodes are copyrighted. Our rights as US citizens with access to public TV include the ability to make copies of aired programs for our personal use. But I wonder how far this extends? Wouldn't making copies of the tapes and giving or "trading" them to others constitute a violation of that copyright?
Regardless, I've found that making copies of television recordings (as opposed to professional quality recordings) usually wind up looking rather crummy, unless the original tape was recorded in SP mode, the broadcast was over a very clear cable channel, and the copy is also made in SP mode. Eventually, all episodes will get air-time in syndication, so it's just a matter of being patient to record them when they come around. And of course, you can wait to buy the commercial releases!
-Gary K.