I don't think that's the case at all. Driving impaired, under the influence of any substance, is illegal in all states
I can drive while intoxicated as long as my blood alcohol is less than 0.08%. What is the safety level on marijuana? There isn't one, so if legalized, one cannot be objectively held as impaired. Subjectivity in the enforcement of laws usually doesn't bode well for the average person. States already face this issue with legal prescription medications. How much makes one impaired? Only alcohol has an objective standard.
Decriminalising a substance doesn't mean that you can do literally whatever you want whilst using it.
The bill as worded, does. While they say it is to regulate it like alcohol, that only means nobody under 21 can buy it. Otherwise, unless its use violates some other law, then you can pretty much do whatever you want whilst using it.
One such law will be child endagerment laws. This bill will not change those and if you grow your own or posses your legal minimal amount as allowed by the amendment, you will still lose your kids and if you are in possession of it near a school and many other places, you will still go to jail and have a stiffer sentence that a mere possession charge.
Those are a couple of the scenarios the bishops brought up in their press conference. As bad a a possession charge is, its even worse for endangering the welfare of a child and related charges. And these are not simple scare tactics, but are playing out every day in Colorado.
That is just one of their objections to the passage of this bill. They aren't encouraging prohibition. They are calling for a better bill for the people of MA. But the media won't let them get that message out and as such, they must resort to paying this large sum to do so.
As Colorado found out. Once marijuana was legalized, any attempts to go back and fix the bill has been met with stiff opposition from various well funded groups. That's why it is important to get it right the first time as it is unlikely there will be a second chance.