i'm not sure on the exact details, so i may well be a little wrong here, but this is how i understand it:
UAC protects the program files folders - so that any attempt to write to them without admin rights will trigger a uac prompt, or in some cases just completely fail, without even giving you an error message.
if you tried installing 'problem' software outside of the program file folders, then you can write files freely, without uac popping up.