People pay the owners of the tv show rights to put them to dvd, run them on as re-runs, etc. Just because they are old episodes doesn't mean that they are free for anyone to watch where ever they want. Even the old ass shows that are run very late at night on networks like Nick at Night are paying out to whom ever owns the rights to it.
Just because you don't understand or don't like it doesn't make it any less illegal.
I hate the fact that the speed limit on my street is only 25mph. That doesn't make it a dumb ass law. It's that way for a reason. Same as why sites like tvshack get shut down. The copyright laws are there for a reason and sites like that are breaking them. Doesn't mean that I don't watch stuff online, hell, I watched the newest Harry Potter from right here where I am sitting now. I just don't cry about it when the sites I get stuff from get shut down, because I know it is going to happen and I know why it happens.