Straight to your questions:
1. The name of the ascxs are hardcoded in the code behind of the aspx (in Page_Load -> AddTab("Customers") which loads Customers.ascx)
This is made just for the purposes of the example. You can use a switch if you'd like.
2. When you click the tab which is already loaded, the ascxs are again laoded
Please check if you have set the ClickSelectedTab property of the tabstrip to true by mistake.
3. If there is any postback in any of the ascx, it loads all the ascx again.
RadTabstrip always reloads all of its pageviews (via the PageViewCreated event of the multipage control). This means the code-behind of your page / UserControls will be fired no matter which pageview is rendered. Loading all pageviews is essential for the server controls living in the page views. If we don't load them every time viewstate will be corrupted and postback events won't fire.
Ben, you should add your controls in PageViewCreated event of the multipage control, but not on TabClick.
the Telerik team