The bullet button is a device that is placed on california rifles to keep them in compliance with California's Assault Weapons Ban. The device replaces the standard mag release button and requires the use of a tool to be able to depress it and drop the mag. Additionally the mag capacity is limited to 10 rounds.
If you found a screaming deal on a model that has a bullet button but do not live in a state that requires it's use, the part could be replaced with a standard AR-15 mag release.
If you do live in California and want o have an AR...well, then... I'm sorry.