It's a bug/weak design in Flash. It can do the same thing on desktop, and does. I regularly go in and kill the plugin process on my desktop linux distro or restart firefox in windows. If you really want to see this, try using Chrome, which gives you a finer comb to identify offending parties. You will find that you have to kill the shockwave process every now and then or your flash game tabs will get sluggish as the bloated monstrosites that Adobe has unleashed on the world leak resources.

I don't think a GUI for kill is a good idea. People not comfortable with a command line can reboot, just like they do in windows. That's safer than handing them a shotgun and dropping them into psdoom.

Edit: However, a shutdown script for the browser to kill flash processes or an icon for a script to do the same is a good idea. Flash bogs down my laptop enough over time that i can imagine it really could chew up a phone battery.