The string value of an "onclick" attribute is taken to be simple JavaScript code. JavaScript includes provisions for labeled statements, so that code is a return statement with the label "jamoscript". In other words, this:

jamoscript: return confirm("Do you really want to hurt me?");

is perfectly legal JavaScript. Labels aren't used much, so they're unfamiliar.

The only context in which the "javascript:" prefix matters is when a URL is expected, as is the case with the "href" attribute of <a> tags.