It is exactly the same as a demolition cut, to allow the beam to slip sideways rather than rest on the beam below.
You will hear many claims on here that it was cut with welding torches during the cleanup by people who do not know this to be a fact yet will claim it is, but as you see, there doesn't seem to be much cleaning up done yet, and there is no noticeable method of access to the steel beam or a platform for somebody to stand on to cut it.
Additionally, you might want to ask yourself what the urgency was in getting into the rubble pile and cutting this steel beam, when the pile is still smoking and everyone else is standing around looking confused. It's got immediate aftermath written all over it.
You can better judge it's scale and the lack of any preparation of the site in this photo, note the firemen climbing next to the beam, the steel is as thick as his upper arm if not thicker ...
[qimg]http://www.fileden.com/files/2006/9/11/214131/pic87932.jpg[/qimg]
Also judging from the thickness of the beam, it looks to be several inches think, you might ask yourself why somebody with a welding torch would waste time cutting it at that angle when the quickest method would be used in a situation like this - a straight line - I think you have mentioned this.
I worked for several years on and off as a welders mate, and though I am not a welder myself, I can tell you that cutting a steel beam as large as this with an ordinary welding torch would not be as easy or quick a task as you might think, and without assistance the cut would not be uniform. I've seen people clean up things by cutting with welding torches and the result is normally quite a jagged cut unless care is taken to produce a straight line - again, no need for that in this situation.
There were several other cuts like this found, these are mentioned in an excellent documentary - 911 Mysteries. Included in that I think, is an excerpt from a TV program where a demolition expert explains why cut's like this are made. I would imagine that everyone in this thread so far is well aware of all this, and could at least concede that it looks like what it looks like.
http://video.google.co.uk/videoplay?docid=-6708190071483512003
In addition, do you consider yourself a skeptical person? I know I do, yet I find on here that it is claimed this is not the case. There seems to be some kind of secret exam that you must pass first, after which you don't have to be skeptical about anything anymore, all you need to do is sit back and take delight in making fun of people who think they are. Odd don't you think?