I doubt many of them think there are no consequences. They have instead come to the conclusion that the positives outweigh the negatives.
No different to releasing game birds. You’re fooling yourself or lying if you claim they have no negative consequences, but you’ve decided the positives outweigh the negatives.
The truth, as is almost always the case, is that any environmental intervention has a mix of effects, and people decide over time whether on balance they prefer the world with the intervention or not.
Currently more and more people are deciding they like an environment with beavers in it, and all that entails. So be it. It will have costs, but so does everything we do, and we find ways to justify or mitigate those costs.
Not really much point getting worked up about it.