Why do I have to swear on the Bible in court when the Ten Commandments cannot be displayed in a federal building?

Why do I have to swear on the Bible in court when the Ten Commandments cannot be displayed in a federal building?

<center><br><a href="http://www.sisodia.in"><img src="https://lh4.googleusercontent.com/-omacfbUs7mI/Tx3nsWpV2eI/AAAAAAAAJBw/ljADS7Pk7V8/s0/Why%252520do%252520I%252520have%252520to%252520swear%252520on%252520the%252520Bible%252520in%252520court%252520when%252520the%252520Ten%252520Commandments%252520cannot%252520be%252520displayed%252520in%252520a%252520federal%252520building.jpg"></a></center>