Not everyone in this world is as nice as the people who run Foundation Flash. Some may want use your hard work to their own advantage by ripping it off and then customising details like the logo at the top, or the colour scheme. You do the difficult bit, for your own web page, a favour or even money, only to have it effectively copied and pasted by someone else. By the time you find out - if you ever do - there could be thousands of duplicate bits of Flash around the internet.
Should I worry?
I would say that you should consider how much you would care if someone stole some of your Flash. How angry would you be? For example, there is little commercial value in the Flash on this site, so I wouldn't go to great elengths to stop people decompiling the stuff here. But in the profession world it can be a matter of financial life and death, preserving your unique identity in a competitive market. Then I would start worrying.
How they do it
Generally, the copying of a piece of Flash in SWF (.swf) form is done by decompliling it. This decompilation turns your .swf back into an .fla file again, ready for you to edit and then re-compile. SWFs are the ones made available on the internet to you and me, whilst the FLA stays in the designer's vault. Or that's how it should be.
Of course, there are legitimate uses for decompilation - usually because the person who designed the Flash has lost their own FLA and needs to modify their SWF for a client. Decompilers are available freely (as in speech, not beer) on the internet, as commercial products. You get what you pay for, and prices range drastically. An averagely powerful one might be Trillix, at $79.95.
How to stop it
Thankfully, there exist programs which can help with this by 'obsfucating' your SWF, and these can be reasonably inexpensive. Particularly good at protecting your ActionScript, they can work in a number of ways: mostly by trying to fool the decompilers. One of the best examples I've seen is the piece of software called SWFProtect, which only costs around 40 dollars, and seems to do the trick. The battle goes on, but people who rely on their code or artwork for a livelihood should consider it a wise investment.
Well, I hope you've found this a useful information piece!