Why do corking fees exist? I've been to multiple combination bars/beer stores, where you can either buy bottles to go, or drink them on premises. If you buy a bottle to go, it costs the base price, but if you buy one to drink there, you have to pay an additional "corking" fee.  Is this due to licensing or tax laws, or just a way for a store to make a quick extra buck?  I know that restaurants will charge you this same type of fee if you bring in your own bottle of wine, but I don't understand why a beer store would charge you this for drinking a bottle of beer that you bought from them.
Reviewing various state regulations on GoBYO, corking fees generally do not appear to be mandatory but instead optional for establishments that already have liquor licenses.

Beverage sales generally represent some of the highest profit items for restaurants.  Soda tends to have the highest margins since it is so cheap, but alcohol is easily a 200% or higher margin and sells per bottle/glass.  If people can bring in their own beverages without paying, or just paying cost, it dramatically reduces the amount of money a restaurant can make.

For a retail beer store that lets you drink on premises, it's possible that they are now dealing with two sets of rules, one for selling and one for consumption, or that they have to take on higher insurance liabilities.  You're also transforming retail space into dining space creating an opportunity cost.  This would justify an increased cost.  It would probably also wreck their general retail business if they tried not charging a corking fee.  If you knew of a bar where you could buy beer at retail cost, wouldn't you go there all the time?