@jagowar
Here's a copy and paste from the Live Mesh help page.
About User Account Control
If you're using Windows Vista, you must have User Account Control (A feature in Windows Vista that can help prevent unauthorized changes to your computer. It will ask you for permission or an administrator password before performing actions, such as installing new software or changing settings, that could affect your computer's operation.) enabled to install the Live Mesh software. To enable the Live Mesh Remote Desktop enhancements, you must be logged on to Windows Vista using an administrator account.
As to your question why? I can at least promise you the intention is not to annoy people :)