The RadTreeList postbacks on node expand because with its current available load mode, the children items of an unexpanded parent are not created until the parent is expanded. This, as previously mentioned, is done as a performance optimization. In case many customers demand the client load mode (similar to grid hierarchy) our developers will consider implementing it for the treelist.
Additionally, keep in mind that in order for the treelist to expand client-side, again its whole structure will be sent back from the server (similarly to the case where you expand the whole control), so there will still be a lot of content to render on the page.
the Telerik team