Since All UFOs can alter their appearance (or more acurately they alter how your brain processes information:

It's important to understand this is happening all the time all over the earth especially in regard to aliens vacationing here from other planets. 

They have no reason to destroy the quaintness of our cultures simply because they also have time travel as well so we are more like Deer here on earth to them than anything else. We are more like Wildlife like Bears or Mountain lions or deer than anything else to most UFOS.

So, coming here is sort of like going to Disneyland to most beings from other worlds.

So, this is what people need to understand more.

If you can alter what people think and what they remember how dangerous would you consider people to you?

None at all unless we let Global Climate changes destroy all life on earth. 

when would they know when to alter people's memories of all these thing?

Mostly I think by what people might say or do but also there are robots that read people's minds so their task would be altering minds when necessary to maintain the quaintness of Cultures on earth.

There would be no reason for war here on earth just like humans wouldn't make war on herds of Buffalo in Yellowstone or Teton National Park.