ask aluminium Forum

I must admit I always thought is was a marketing strategy. It's not really convenient to keep a bottle without the bottle cap. Bottles without a cap might be consumed faster than with a cap, possibly boosting sales. Of course liquid can be poured in plastic or paper cups instead, but this is less convenient to do than just hand out plastic bottles. Unfortunately I  haven't found anything that supports this initial assumption, except for the following article covering a part of the profitability of plastic bottles and the reason why it might be hard to ban them completely or replace them with paper cups.

    They are profitable for stadiums because  concessionaires can dispense them more quickly than beer poured from a  keg or can. Also, at $5 to $6 and more, the 16-ounce and 20-ounce  bottles include a  nice mark up, experts state. Fans like them because  they are easier to carry than plastic cups and lack the suds of draught  beer. 

As mentioned by Stephanie Vardavas the main consideration for removing bottle caps are safety issues, i.e.  to make them less effective as a weapon, as is mentioned on several sites as well.

E.g. at a game of the Cleveland Browns against Jacksonville Jaguars in 2001 there was a bottle-throwing incident, referred to as "bottlegate", where fans got upset with a referee's decision and started throwing bottles. The Giants, another football team,  subsequently decided to ban bottled drinks

To add something to Stephanie Vardavas answer, in Argentina and other South American countries they don't even give you the bottle. They pour the soda/coke (no beer/alcohol in stadiums) into a 500ml plastic cup, making the process painfully slow.

they will be less appealing and less effective as missiles.

A full bottle of liquid (even a plastic bottle) is heavy and could injure someone if it were to be thrown and hit them. But a plastic bottle without a cap, even if it starts out with some liquid in it, will be lighter by the time it has traveled through the air for a few yards.

This mystified me the first time I experienced it (at Old Trafford). But it makes perfect sense once you think about it.