That's a good question.
You could ask the author of the book on private prosecutions in Canada, who is seated to my right and who could probably answer better than I can, but I'll take the first cut at it.
First of all, the purpose of a prosecution is primarily to punish and deter. The purpose of an environmental bill of rights action is really to restore and clean up, so the two have different purposes.
You are right that in common law and in statute the crown has a right to intervene and stay any private prosecution. This bill, on its face, would not attempt to change that. You could add a clause doing that. I won't get into the policy arguments for and against that. There are good ones on both sides, I think, but the drafters have chosen not to include it.
But what this does, though, is provide a safety valve power. These suits, called citizen suits, usually get used in two circumstances. One is in offences that aren't serious enough to warrant the crown using up resources for a full criminal prosecution--the Syncrude duck kind of problem and those sorts of things--but which nonetheless are cumulatively very serious in terms of their environmental impact. It allows citizens who want to bring those actions forward to do so, but appropriately, I think, they should have relatively small penalties and allow for restoration and compensation.
The larger cases, the really serious environmental offences, should continue to be dealt with through proper prosecutions.