A disclaimer first. I have no view on the current candidates for tenure and promotion at my school or anywhere else except at my school I have no reason to think they are not worthy of promotion and tenure. This post is about a process, not individual people.
Ok, having dealt with the niceties, the tenure and promotion process everywhere I am familiar with is a sham -- another way to keep the privileged in their privileged positions. In some respects it is the big version of partner hiring in that partner hiring (when both would not be hired without reference to the other) is nothing more than cutting in line. I'm not going down that path having had my say much to the displeasure of the faculty spouse crew. And, by the way, the cutting in line does not even touch the complexities that can arise later.
Back to tenure and promotion. Some committees evaluate and some just gather information. But from experience I can attest to the fact that even the information gatherers can influence the outcome by the referees chosen. That is, if they do not let the candidates pick the referees which is something we once did.
So you have outside and inside letters reviewing scholarship and reports of those visiting classes. Let's get rid of the class visits first. Most are preannounced so as not to actually see the teaching that takes place when an observer is not there. (We would not want that now would we?) In 30 years of teaching I have not seen a single negative review of teaching. Why is this? Lots of reasons. First you do not want to hurt someone's feelings when deciding to ask the tax payers to give them a life-time annuity. Second, these are your pals and paldom overcomes objectivity every time, And, if you write something negative and tenure is granted, you have an enemy for life. Class visitations are a time-wasting formality.
The inside letters of evaluation fall in the same category, In 30 years I have seen 2 letters of maybe 400 that could be viewed as kinda/sorta negative. Yes, nearly every reviewer of every article ever written has found that the work "meets our standards." If that were actually the truth, time to reevaluation those standards. The reason for the inside letter charade? Refer to the paragraph above. And, you can add, elites evaluating elites. Or, frat boys evaluating frat boys after a short period of law school hazing (requiring the writing of law review articles that are largely irrelevant.)
Then there are the outside letters.I have seen hundreds of these too. There is a slight possibility of a negative letter here but so many things stand in the way. First, of course, is the market for letters problem. Yes there is a market for letters and if you are known to write negative ones you are less likely to be called upon again. It's not a business but some people like being regarded as an expert. Like an expert witness, testimony that does not help is not welcome. And, "help" is support for what the faculty wants to do. Second there are the letters involving areas of law that have a definite "correct" point of view. You know those areas. So, if you are an expert in one of these areas you do not respond negatively to an ally no matter how bad the work. After all, they are on your side in the ideological battle. Third, if you write a negative letter you just gum up the works. In all likelihood the faculty is going to grant tenure and promotion (they almost always do) and a thoughtful negative review will likely be ignored and conflict with the internal reviews. I have seen this work repeatedly in the rare instances in which the outside review is negative. I recall one letter in particular by a reviewer who noted that the new articles for a tenure candidate were pretty much the same as the articles for promotion. In other words. "Duh, there is nothing happening here." This was, of course, not even spoken of. Basically, unless you have principles and are an idealist there is no upside to writing a negative letter.
So, why do all this?