Real life example from Norway:
1: Police wanted/suggested a filter to stop child porn (think of the children, etc)
2: This filter is then implemented by the ISP`s that wants to use it.
3: Police/Government suggests/mentions that it should be required for all ISP`s.
4: Government starts talking about implementing filter for stopping sites that "can be bad"(read: piracy sites)
(what will happen in the future, at least as i suspect it)
The child porn filter will be forced upon all ISP`s, it will be changed to also block other sites that the government/police thinks that should be blocked.
Now this filter is as of today easy to go around by simply using other DNS servers, but the problem is that censorship is wanted at all. I will not be surprised if the next step is to put the filter on a lower level on the network stack. And even in the long term, forbid the use of TOR/Freenet etc.
You're implying that censorship will lead to a slippery slope but we're already sliding down one.