On Ancestry.com you can only see non-free databases and other trees with a paid account. If you can't see the trees you can't exercise copying options. Copying from other trees is nearly always a very bad idea anyway.

On Mundia.com, a mirror site with public trees from Ancestry.com, you can see other trees (while it is still a free 'beta' site) but there are extremely limited tools for making changes to your tree.