Macbook finder copy changes permissions to 777
After some experimentation, I figured out that the USB drive was formatted FAT, and when Finder copies to a disk formatted FAT, the permissions get changed to 777.  When I reformatted the USB drive to MAC OS, then finder copied the files with permissions correctly preserved.

Side Note: Someone suggested the permissions don't really change to 777, but rather FAT simply does not support unix permissions.  HOWEVER, I would point out that I first noticed this not when I initially copied my files to the usb drive, but when I copied them back to my macbook from the usb drive, and then I saw that on my macbook the permissions were now 777.  So, in effect, FAT's non-support of unix permissions actually did change the permissions to 777.