Um, because that is his salary, and it impacts what else they can do with the roster. Because people are talking about his worth/value to the team, and that has to be measured against his salary. Value has a cost, and his cost is a really big number. How should he be measured, if not against what he costs? How about against other catchers? Even then, if he was paid $1MM, last year would have been a successful year, but he's not paid $1MM. So how should he be measured, if not by his contribution relative to his cost? Isn't that how value is determined?
For example, if he stayed where he is in WAR right now, how would you judge him, if not against how much he costs relative to that WAR? Because for $5MM, he's a bargain...should we discount, say, 8MM a year for being a hometown boy (which he's not, actually), meaning we should judge him relative to $15MM per year?
Because you can't just judge him relative to other C/1B/DH, without understanding the cost of that contribution relative to what other players produce per dollar spent.