Governments can and often do check DVDs in the mail and in stores and confiscate any material that offends them and perhaps prosecute the owners and the sellers of the DVDs.
But the government has a lot less control over the content of the internet. Because the source of the material usually is located in a jurisdiction where the porn is legal to make and to have. And there isn't any easy way for the government to spy and to prevent people from other jurisdictions from connecting their computers to the source and getting the porn they want to see.
Although some governments are now passing laws that force internet providers to spy on the content their users view and report that information to the government. It won't be long before the police and other government agents will be bursting into people's homes with warrants to confiscate and search their computers for illegal material. Because these agents will be getting their information straight from internet providers. And they will have enough evidence to get search warrants.
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