By definition buggy websites that crash the browser are bugs in the browser.
It may have security implications, or it may not. It might just be an innocent case of someone using assertions instead of proper error reporting. Nevertheless it's a bug in the browser.
No. Safari chose the exact wrong way to handle this case. Let's suppose some webpage is in fact allocating too much memory. It is the user agent's job to inform the user of this fact. What does Safari do? It silently crashes. It's not even about displaying the wrong error message here: the handler for the crash is to simply refresh the page and render it again. But this is exactly the wrong way to handle out-of-memory errors: chances are the web page will again allocate too much memory and crash yet again. In the end the final displayed error message is "a problem repeatedly occurred" with no reference to the nature of the problem.
I hate this trend of hiding error messages from the user. Apple as a company known for its attention to detail in UI, should have been the one company especially dedicated to presenting a good error message without overwhelming the user with technical details—it is supposed to be the master in user communication. And it is not. Hence my disappointment.
It may have security implications, or it may not. It might just be an innocent case of someone using assertions instead of proper error reporting. Nevertheless it's a bug in the browser.