They have most likely gotten your info from GMAC (have you taken the GMAT yet?)
An application fee waiver is often an easy wait to entice folks who are a "good fit" to apply to a school that they would not have applied otherwise (meaning they were not one of your 5 chosen schools).
GMAC sells your records and often schools can "target" folks who fit their profile but did not apply to the school. Just wait till big packages start rolling into your mailbox. The barrage has begun

P.S. It is a good thing. You just have to consider if you have the time and energy to submit another app and if this is the school you would be interested to attend, as the golden rule of application states - apply only to schools you are actually willing to attend.