Before issuing warning the HR has to follow principles of natural justice.
4th July 2017 From India, Thane
As the misconduct is not serious then they have issued warning, suggesting you not to involve in such incidents / not to repeat such incidents, warning letter is almost an advise.
If you have any concern on the same you can submit your comments with reference to the warning letter so that the HR can organize the Domestic inquiry.
As it is not a capital punishment, in general HR is authorized to issue a warning letter. Even in most of the companies HR is authorized to issue capital punishment letters also.
8th July 2017 From India, Hyderabad