i've always wondered this too. The logic of "i am not required to take this test" could be like pleading the 5th. It winds up making you seem like you have something to hide.
Court logic: "If he wasn't UI, then why wouldn't he submit to breath test/FST?"
This is why i don't drive drunk. Too complicated. I think with pot, if it can be proven you were toking in the car or very recently (i.e. smells like weed/visible roaches/possession of pot/paraphernalia in the open, etc...) it should be considered a DUI. Sorry, but thats the way it is. Common sense on the LEO will be necessary. Just like if you'd had a glass or 2 of beer, you can openly admit that. If you have an empty bottle in the backseat or open container you're fucked.
This all clouds the issue. A DUI is a DUI. The legalization of sales/distribution/cultivation won't change any position on DUI.
Me and a buddy got stopped once in socal a few years back. Had just smoked a joint. The cop stopped me for speeding. We had a box of swishers on the dash.
The cop asked why we were in a hurry. Blockbuster box also on dash. Ding ding ding.
Me:"Officer, we are in a hurry to return the video before blockbuster closes..."
Long shot.
Officer: "Well slow it down. A speeding ticket is a lot more than a late fee"
lol
Me: "I'm sorry officer. I'll slow it down."
Officer: "No problem. And you probably shouldn't be smoking pot in the car. Have a good night."
We shit our pants and drove away. 5mph below the limit...
Some LEO's really know how relatively harmless pot and potheads are. The focus needs to be on hard narcotics. Legalizing weed would help make that happen...