Why do they say that you cannot saw treated pine without voiding the warranty - surely it defeats the purpose of buying timber, doesn't it?
The reason for the treated pine warranty being voided if you cut it is that the treatment is an externally applied process and doesn't go all the way through the timber.
It's sealed from the outside, so the bad bits can't get in.
If you saw the timber you will expose untreated areas of the timber to all the things that would like to eat it or grow in it.
If you cut treated timber, apply a wood preservative to the cut faces. This will prevent any problems.
PS: Do not burn offcuts of treated pine as the fumes and residue are toxic (definitely not one for the BBQ).
You should also wear a mask when cutting it and wash your hands before eating food.
If you are using treated pine to make a picnic table, make sure you paint or seal it.