Unresponsive menu button on several Android apps
This menu button is a backwards compatibility feature. It's displayed on all apps that say they are designed for a version of Android from when Android devices had physical menu buttons. (That's Froyo or earlier.) The Android system can't tell whether such an app needs the menu button or not, so it has to display it to make sure the app is usable on newer devices without the hard button.

If the button annoys you, contact the author of the app and ask them to update their "target version". They can do this and still be compatible with older versions of Android.