But the effect is the same - their society would be absorbed, and probably forgotten.

It's like time travel - you can't go back because you don't know what kind of effects you'll have on the future. In this case you don't know what kind of effects contact will have on their (the tribe's) future.

So how about Japan for example? Do you think contact with the west was bad for them?

you're comparing different things, or do you think that a highly advanced CIVILIZATION like the japanese encountering the west is similar to a TRIBE of HUNTERS-GATHERERS.... damn!

No. I was referring to messages by lwhi and nitrogen. As nitrogen said, there are less invasive options to contact them than colonization and there are cases where it is beneficial to the (in some ways) less advanced civilization. It might not have been fun for the japanese when the westerners first came there, but in the long run it was a good thing. And while the japanese culture was certainly changed, it was not destroyed or forgotten.