do they not let members use the internet?

They are allowed to use the internet, though they are advised to stay on "church approved" sites, and if they surf around and see negative information of the church to assume it is all lies put out by enemies of the church.
They do assign people to different message forums if they become aware of them. You can tell right away as they will always focus on the accuser being a "loser" or a "has-been" or "crazy" and that they are saying these things simply for publicity. You can find a lot of them on Anti-Scientology You Tube videos.

🚜👦Tractor boy...got you in my tractor beam!👦🚜

They do but the members are warned about what they will find. They are told anything negative is a lie. I read about this in a book written by the niece of the head guy in the religion. Her book is very eye opening and shocking.

The Church of Scientology can't do anything to regulate the lives of its regular members, only the Sea Org. In the Sea Org, you have to pass a few security checks if you want to be able to use the Internet on your time in your apartment building (all Sea Org members except the most senior execs live in church-owned apartment buildings, usually shared accommodations except for married couples and even the senior execs live in townhome-style condos) or if you have a smartphone/tablet instead of a computer.

With non-Sea Org Scientologists, they can't impose any kind of rules or restrictions. If a Scientologist does go out reading anti-Scientology material and brags about it, the most they can do is suspend you from church services, with the most extreme penalty being excommunication. But if a Scientologist is reading that kind of stuff and admitting to it, they're already on their way out, whether they realize it or not.

Most Scientologists have read the stuff thats out there, they're not as unaware and gullible as anti-Scientology types (who are extremely condescending and holier than thou, just like the Scientology fanatics) make them out to be.

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THE CULTIES that do use the internet are by and large from OSA (their office of special affairs)and these people are told to prowl the internet and websites and post either positive things about the cult or to disparage any and all negative comments, and they will never admit they are scientologists nor will they ever answer any questions posed to them in comments, like a broken record they are... a telltale sign is when they never respond to any questions or criticisms,repeat posts after posts saying literally the same exact things, when they post "i'm not a scientologist, BUT...." and so on..

Christians and Muslims also have pretty much unfettered internet access.

While it helps some see the world in perspective, many just double down and use it as a recruitment tool.