The thick plastic corners are pre-drilled at a angle to index the included 3 1/2 inch wood screws.
Can see the logic in stabilizing the table instead of screwing them straight into the 2X4's.
I used liquid nails and along with the 3/4" hardwood plywood, the bench seems to be pretty stable.
We'll see..
Thanks everyone for comments, pictures and tips.
Great web site!