system
Given the following post, respond as if you're commenting on Reddit:
human
[Harry Potter] Why isn't the pensieve used in court?

Taking out a person's memories isn't very hard so most crimes could be quite easily solved by just having a look at what the alleged criminal remembers. For example had the ministry done this, Sirius wouldn't have had to go to Azkaban.   
Even outside of criminal justice there is a lot of use for the Pensieve. Harry could have proven that Voldemort had returned by simply taking out his memory and showing it to those who doubted him.   
And sure, people can change their memeories just like Slughorn did, but the memory-editing is easily noticeable and probably not that easy to do in the first place.
gpt
I don't think you can't forcibly take memories. If you could why would Dumbledore have gone through all those hoops with Slughorn if he could have just taken it while Slughorn was sleeping.

If people have to consent to giving their memories then there's no way to control someone altering their memories. Even if you can tell it's been altered you can't get the real memory.