Actually, I'm sure that someone has tried to print their own Dragonball shirts in the past. I'm sure Funimation's lawyers jumped on them. I didn't research this, but I'd bet on it without hesitation. If they wanted to sue us for selling shirts with their characters on them, proving actual damage would be trivial (even though damages wouldn't be worth suing over).
I'm no copyright lawyer, but I do have some basic knowledge of the topic.
Copyright by definition is the right to reproduce an existing work. All the anime pencil boards and trading card games are reproduced under direct authority of the copyright owner. You may have heard about something called "fair use" but this only applies to things like parodies, editorial use, and other very limited venues. A shirt with a Dragonball cel would easily be mistaken for a Funimation licensed product.
Putting a copyright symbol after 1972 (or close to that) is no longer required by law to declare a copyright. Copyright is created the moment an intangible idea is rendered into something tangible. We're allowed to talk about our cels and show pictures that represent that tangible object. You could even sell your commentary about a cel. Laws after that are case by case.
Yes, I paid a lot of money for my cels, but I pretty much have just that cel, not the permission to reproduce the character.
You may be thinking, "Museums make posters, why can't we?" The answer is that museums own every aspect of that piece of art. The buck stops with them. With cels, the buck doesn't stop at the owner of that cel. Bucks stops at the owner of the character(s) represented in that cel.
Fan art, fan cels, fan fiction, and fan subbing are all technically illegal in the US if there is an American license holder for that piece of art. Why doesn't Gene Rodenberry lay the smack down for the annals of bad Star Trek fan fiction written every hour across the Internet? Well, that's like scolding diners in your restaurant for taking too many napkins. Or to bring back my museum example, it's like Campbell's Soup suing whichever art gallery is in charge of making reproductions of that Andy Warhol illustration from the 60s. It's bad business.
As long as no money is being traded by the rest of us non-owners, then most corporate types usually shrug it off.
Random googling for some fascinating reading:
On degenerate art from preexisting copyright:
http://www.wired.com/news/culture/0,1284,55592,00.html
Copyrights on art restoration: (google cache link because page is dead)
http://216.239.41.104/search?q=cache:pFLGXMefraMJ:palimpsest.stanford.edu/waac/wn/wn03/wn03-3/wn03-302.html+art+restoration+copyright+law&hl=en&start=3&ie=UTF-8