Bottle tops are removed as with the lid on, a bottle can be used as missile. Without the lid, the bottle will have less impact as some fluid will be lost in flight. Also, bottles with the lid on can create a trip hazard, as in the event of an evacuation or other incident, a bottle with the lid on will roll, whereas a bottle with no lid on will crush safely.
All bottles in the arena are plastic, there is no glass allowed into the bowl area. Beer bottles do have the appearance of their glass counterparts; however they are a soft plastic. All plastic disposed of on-site is recycled and all surfaces in The O2 are treated to be non-slip, even when wet.
These decisions are always made by weighing up the overall safety benefit and acting accordingly.