Honestly, it's part of the simplicity vs. features tradeoff, where we generally tend to side with simplicity and ease-of-use whenever possible.
Since all of the scripting that makes our cart work is loaded into your page on-the-fly from our servers, any such scripting as you suggest would never load in the first place if our servers can't be reached. The only code we provide to you for copy-pasting is the minimum possible code necessary to call upon scripts from our servers and make the cart buttons (and any corresponding menus/text-boxes) appear on your pages. The only way around this is to add something to the code that you have to manually paste-in on your own pages (which you can certainly do, or hire someone to do, yourself), and that frankly starts to open a whole can o'worms if we start providing that sort of solution.
Simplicity and ease-of-use is our core mission (with apologies for the corporate buzzspeak :^), without which we'd just be yet another complicated, feature-bloated, bug-ridden, hard-to-implement cart -- of which there are plenty on the market to choose from, if that's what you want. If we tried to be everything to everybody, there'd be little left that distinguishes E-junkie from any other cart solution, while compounding the complexity that becomes harder for a small team to maintain, harder for our Merchants to implement on their sites, and increases the likelihood of conflicts/bugs/crashes/failure. If we have to let one Merchant go elsewhere in order to keep the majority of our simplicity-loving Merchants happy here, that's a business dilemma we have to accept.
As part of this business strategy, we intentionally keep most of our scripting code on our server, so we have centralized control over how it functions and can provide the most minimal-possible codes for you to copy and paste-in for our cart buttons, without requiring any ongoing maintenance or remediation on your part to "keep up with us". Besides centralizing our ability to offer improvements and bug-fixes, this also minimizes any chance for mistakes or conflicts when you are implementing our cart on your own sites' pages (you wouldn't believe how many people need assistance just copy-pasting our minimal code as it is already); whenever we fix a bug, improve something, or expand functionality, it instantly becomes active for every single Merchant without them having to do a single thing. In other words...
"It's not a bug; it's a feature." :^)