I would suspect that it's the fault of ADB. Perhaps the mouse data protocol used over ADB only supports reporting of a single button type, or maybe it just reports it in a way that OS X doesn't recognize as a genuine second button. See if you can get Kensington Mouse Works to treat the button as Control-Click. That's probably your best bet.
You can have my iBook when you pry it from my cold, dead fingers.
iBook - The computer of choice for the enlightened CS major. Come on Apple, let me do a commercial. ;-)
"An alloc a day keeps the DRAM away!"