The methodology pertaining to trials is the user installs...and trials the software...and while doing so has friendly reminders that it is a trial and to purchase the full version if inspired to do so.
After the expiration of the TIME limit [60 days] a trial program will generally cease to function.
If you uninstall/reinstall you simply miss some of those reminders.
Neil has suggested there will be a revision to ensure that once expiration is reach there remains a clear indicator of that expiration....for those who 'miss' the messages by doing such things as multi-installs....