I think the strongest protestors to this forum have lost people to suicide. They likely don't want to accept that it was a choice that their loved one made either freely or rationally because, it means accepting that they chose to abandon them.
Their loved one may also have been suffering from a mental illness so, the 'professionals' out there may be keen to insist on the idea that they didn't have proper mental capacity and could be negatively influenced by a forum such as this. Either in terms of more passively- being in a negative echo chamber or, directly- they may believe they were encouraged to do it here. (Which doesn't happen- in my experience.)
Either way though- I suspect they see the forum as the equivalent of handing a loaded gun to someone who isn't entirely in their right mind. I imagine one of the biggest concerns/ criticisms is access to method information. They fail I think, to properly acknowledge how difficult it then sometimes is to obtain methods.
It puzzles me a little how they square such mental incompetency, unorganised thought and impulsivity with someone who manages to join a website, research, source everything needed and follow a sometimes fairly complicated protocol to suicide.
I have very mixed feelings about it myself- if I'm honest. While I'm grateful for the information myself. While I believe sound minded adults should be able to make the choice for themself, the bigger problem is the internet itself. How do you reliably gatekeep potentially lethal information? The answer is- you can't really- even if access was officially age restricted. We can't assess competency here entirely. But then, it shouldn't need to be up to us. Assisted suicide should be legalised and regulated.
Basically though, people who see suicide as an evil or even totally tragic act with no justification will see forums that don't actively oppose/ discouage it evil too.