Some beer choices after a round are more obvious than others. Had my first Red Stripe on a cruise to the Bahamas, where they way overcharge you for a beer, but you pay anyway. More context below.
The joke is based on the bartender offering beer choices to a customer who happens to be a golf ball. Since golf balls are associated with the brand name “Titleist,” the bartender assumes that is the only option and brings one specific beer instead.
It plays on brand recognition and the idea that some names become so common they replace normal choices. The humor comes from treating the golf ball like a regular customer while also treating its “brand” as its personality.