xsi5: editing instances in xsi

In 3ds max when I make an instance copy of an object, if I modify it I also modify the original object.
How can I do the same thing in xsi?
I've tried to duplicate as instances and I can't edit the instances at all.
Are they freezed or something?

In xsi you can't edit an instance of an object because it's just a null that represents the position of the copied object.

Strange
Why would they do something like this?

Because it's very light, resource wise.
And it's possible to make thousands of instances of complicated objects without actually hanging your machine.