How to clear stack back to root activity when user leaves application?

Question

I have an application with 2 activities, LogonAct and MainAct. LogonAct is a logon activity which I want to force the user to go through each time they return to the application. I've set android:clearTaskOnLaunch="true" on LogonAct.

When I first start the app I go through this sequence of screens,

Home -> LogonAct -> MainAct -> Home

I then follow this sequence,

LogonAct -> Back -> MainAct

Why is it bringing me back to MainAct? Shouldn't that activity haven been closed since LogonAct has android:clearTaskOnLaunch="true". I expected to be brought back to Home when I hit the Back button from LogonAct.

Relevant snippets from AndroidManifest.xml,

   <activity android:name=".LogonAct"
             android:clearTaskOnLaunch="true">
       <intent-filter>
           <action android:name="android.intent.action.MAIN"/>
           <category android:name="android.intent.category.LAUNCHER"/>
       </intent-filter>
   </activity>

   <activity android:name=".MainAct">
       <meta-data android:name="android.app.default_searchable"
                  android:value=".SearchResults" />
   </activity>

I'm using 1.5.

Any help appreciated.