I think they do understand that. It is, however, nontrivial to edit compiled JavaScript. There might also be legal complications to editing the scripts...
If you click to include omitted results, the number of results stays at 631k when you get to the last page they will show you.. (1000 results max) Though I don't think it would be hard to imagine that the actual number is far fewer.
Security concerns was not the only argument.
"It's clear that JavaScript is a very powerful and useful technology in the right hands. Many free software developers have written add-ons and enhancements to popular websites thanks to tools like Greasemonkey. There's a slew of fantastic free software Greasemonkey scripts for Gmail. The existence of scripts like these shows both that Gmail's JavaScript is not trivial, and that there are users who could make useful, interesting contributions if the JavaScript were released as free software for them to modify."
They argue that we should be able to change GMail's javascript code just like we do other applications we use.