When it's not, we get problems like this: Mr. Marcos Velasco from Brazil is completely openly writing viruses and making them available from his website to anyone, anywhere in the world. Apparently this is not illegal in Brazil.
So any kid, any lunatic, any anarchist anywhere can download all his viruses complete with sourcecode and do whatever they want with them.
And Mr. Velasco has no problem with this. In fact, he has just given an interview about his activities to a Finnish magazine ITViikko. The interview has been published in English on mobilemonday.net.
Writing viruses is wrong. Distributing them is even worse. It should be illegal, too.