I think it’s because they think some of this one is actually going to make it through the atmosphere without burning up. Also, they won’t know until the last few hours where it might land and they have no control of the re-entry. When you think about it, landing in somewhere like Times Square or a heavily populated area would be a pretty big disaster if it actually gets through. Odds are against it though.
RE-ENTRY UPDATE:
NASA has issued an update on the condition of the decaying UARS satellite:
“As of 1:30 p.m. EDT Sept. 21, 2011, the orbit of UARS was 120 mi by 130 mi (190 km by 205 km).
Re-entry is expected sometime during the afternoon of Sept. 23, Eastern Daylight Time.
The satellite will not be passing over North America during that time period.
It is still too early to predict the time and location of re-entry with any more certainty, but predictions will become more refined in the next 24 to 48 hours.”