A counseling statement can be given for just about any reason. There is nothing that says it has to be reasonable. However common sense dictates that it should be reasonable and it must be factual. Based on the information you provided, you placed yourself in a position in which it was reasonable for leadership to assume you were drinking an alcoholic beverage. The leadership apparently chose not to confront you at the bar potentially to avoid an Article 15 situation, if you were drinking beer.
By making an observation of the incident and documenting it in a counseling statement they were firing a shot across the bow. They are giving you a warning, that should you place yourself in that position again they will most likely confront you on the issue and proceed with more stringent enforcement.
If in fact you were not drinking an alcoholic beverage you have the ability to disagree with the counseling statement and provide your own statement in your defense.
I would suggest you consider avoiding places which serve alcohol and avoid any perception of drinking alcohol. Remember the leadership can give you an order not to enter these establishment, they can (through the commander), revoke numerous privileges (pass, alcoholic, tobacco, visitation, MWR, etc). When the command starts counseling you or revoking privileges you can bet they are watching you and any violation will most likely result in an Article 15.
This action was most likely the chain of command giving you a break. However they may have thought they had enough to proceed with an Article 15 based on their observation but later found out thorugh JAG that they could not proceed with an Article 15 because they failed to verify the beverage was alcoholic. Either way this is a freebie. Should you want to disagree with the counseling I have provided some links to other post you might find useful.
Other Post You Might Find Useful
How do I tell my side of the story? http://asktop.net/q-and-a/how-do-i-tell-my-side-of-the-story-on-a-counseling-statement/
Should I refuse to sign a counseling statement I do not agree with? http://asktop.net/q-and-a/should-i-refuse-to-sign-counseling-statements-i-dont-agree-with/
Do I have to allow a Soldier time to respond to a counseling statement? http://asktop.net/q-and-a/do-i-have-to-allow-a-soldier-time-to-respond-to-a-counseling-statement/
How much time can a Soldier take before signing a counseling statement? http://asktop.net/q-and-a/how-much-time-can-a-soldier-take-before-signing-a-counseling-statement/
Should I refuse to Sign a Counseling statement? http://asktop.net/q-and-a/should-i-refuse-to-sign-counseling-statements-i-dont-agree-with/
I hope you found this information useful and informative