And where the interface is screwy in Safari, that's due to a web-site designer choosing to focus on making something work in IE (the number 1 browser), and be damned anyone else. That, or using MS FrontPage to design the page (*shudder*).
If you're being *told* by the website that it's not going to work in Safari, it's likely that the designer couldn't be bothered to test against Safari and so just made a fail-safe. You can fool websites into thinking you're IE by turning on the Debug menu in Safari (there are a number of ways of doing this).
The other reason is ActiveX a Windows-only, IE-only proprietary technology at the root of almost all the security flaws in Windows IE. It's an interface such that websites can arbitrarily load and run applications on your computer in an unsecured way. Naturally, many people think this technology is the devil, but other sites (such as, funnily enough, Windows Update) require it to function.