You can't take any meat product and dairy product.
I think they are just doing their job. As long as you don't take any prohibited items back and report everything you have purchased, they are fine with it.
I used to cross the border everyday. np for me
btw they are supposed to ask you the nature of your trip because business and personal travel probably lead to different questions afterward.
It was okay for me. For going back from Seattle, they mostly just asked me some standard questions and so far so problem at all for me. For going back from air, they asked more questions but still no problem for me.