Percentage-wise, yes, but not in absolute terms. A $2.00 soda probably costs the bar $0.05 in syrup and labor, for a markup of $1.95. A $5.00 tap beer probably costs the bar $1.00 total, for a markup of $4.00. Markup on cocktails is even more - booze is where full-license restaurants make their money. Margins on food tend to be razor-thin, when you figure everything in.

Also, most boozers at a hotel bar will have more than one, whereas the soda drinker will very rarely purchase more than one unit. I always keep that in mind if I'm not drinking. $10 for a club soda and two hours at the bar, keep the change, will ensure that the soda stays full and that the bartender doesn't begrudge you, and is still a hell of a lot less than I'd spend on alcoholic drinks. Each seat at a bar, just like each table in a server's section, represents potential income. 20% of my two-dollar tab isn't much in comparison to 20% of the tab for a guy who downs four martinis and then buys drinks for the ladies on the other side of the bar too.