Why does the ports system delete distfiles?

[Alan Corey]

I've seen this before, I wonder if there's some environment variable I 
can set to stop it?

I try make fetch on a port, it fails due to a bad site.  I hit Ctrl-C to 
stop it, it goes to the next site and downloads the file.  Then it deletes 
the file when it finishes.  I type make install and it tries the bad site 
again...

  Alan