I’ve always found it unfair coffee is generally free at any employer but soda or flavored water is not.
Definitely more equitable to offer it for a reasonable price and just pay people slightly more (yeah yeah I know what you're going to say).
Coffee is a lot cheaper and has a wider audience.
A 12oz soda from a soda fountain has a marginal cost of like $0.10. Even then cans of soda can be had wholesale for ~<$0.50, especially off-brand varieties.