It's true that you don't have to accept the bonus. You still get your regular pay, etc. Then after 1 year, it is prorated. So if you stay a year and a half, you would have to pay back 1/4 of it. (it's prorated by the month)
If someone isn't sure they're going to stay the two years, the prudent thing to do would be to park it in the bank until you've qualified for it. Since I need to get the 1000 hrs sic time, I'm not going anywhere early so it was a non issue for me to sign it.