Edit: I am taking a break now and will return later this afternoon/evening to respond to any comments and answer any questions. Thank you everyone for a great and engaged AMA so far.
For example, you are allowed to withdraw your application to enter the US and leave the preclearance area. Additionally you are bound by different laws for search and siezure.
At this point I am only going to the US through preclearance areas because you are still on Canadian soil and bound by Canadian laws. That doesn’t necessarily mean that USBP won’t break those laws, but the Canadian courts get to decide if laws were broken instead of the US courts.
One thing I have not found a straight answer on yet is if USBP can compel you to unlock your electronic devices in a preclearance area. My current strategy is if I am asked to do that in preclearance I will withdraw my application to enter because Im not letting anyone access to my phone.
For example leaving the Schengen area it's obligatory to go through "exit immigration" and get your passport stamped. Leaving the US, you show your passport to the airline, but usually there is no formal immigration check at all.