"...I have a question....
WHY would you want to chop the tree down AT the garden center or nursery"
Just to see the look on the nurseryman's face when you finish sawing a $200 maple in half...
Seriously, it's not really l"chopping a tree down" in the sense you're chopping it to the ground. Trunk chopping in bonsai is step one in bonsai training. The majority of bonsai begin as larger trees that are reduced to bonsai height by removing their upper growth.
Bonsaiable trees in a nursery or in the wild are sometimes 25 feet tall or more. You can't get a 25 foot tree into a pick up, much less a four door sedan's trunk. You have to reduce those trees to three or four feet not only to make them fit into the trunk, but to start them on their way in bonsai training.