There's a terminology issue here, I believe. This is how I understand this grey area to work (but I am as fallible as anyone else):
- An ATC unit cannot issue you with an INSTRUCTION in class G, as that would mean that you would be obliged to always obey, which clearly you can't if you are in charge of your own navigation under VFR.
- An ATC unit can issue you with a REQUEST in class G, which you are obliged to acknowledge. You are free to accept or decline such a request, but you should not just ignore it, as has already been pointed out.
It seem that this principle is generally poorly understood, which in itself is worrying. I've long felt that the growing complexity of the rules framework that we operate in is to blame for this - it's unreasonable to expect any of us to fully understand the plethora of rules upon rules upon rules that we now have.
VP