Moderating a forum is not that easy you know, in general it is a voluntary untrained thing, done in ones own time (do you work in the evenings for free?) and done with very few guidelines.
Who sets up a forum and writes a manual for what the moderators should/must/must not do etc? So they have to make their own mind up what is offensive, what should and should not be said on a public forum based on their own lifestyle etc.
The general idea is to keep threads from going off topic and being offensive to others and by others you have to consider everyone else that can read the forum at anytime of the day or at a later date.
The moderator has to make a decision on the spot (as there may not be a lot of time before someone else posts to it) so what decision can they make? Delete the post, delete the thread, lock the thread.
Deleting a single post can affect the entire thread, deleting the thread can be done, especially if it is against the aims of the site, locking a thread can simply be used to prevent any further posts in the same ilk but leave the original post & answers.
If you continue to question the moderators in the open where everyone can read it this belittles them, reduces their authority, makes the forum look bad and that reflects badly on the site so expect threads like this to be locked or deleted without any written reasoning given.
A really good moderator will make decisions that keep threads running that do not breach forum rules and general good etc, allowing little banter but not outright offence then if any posts are deleted do the follow up PM explaining why and asking the poster not to do it etc.
However, remember what I said above, doing all this on a busy forum takes a lot of time and in the evenings they have better things to do, during the day they have their day job to keep up with so a compromise is usually taken and the quickest thing to do may to be simply delete or lock without notifications - especially when it may be obvious why.
And if the person thats being had a go at reports the post and says they are offended then they have to make a decision of delete the posts/thread or not. Its not always such a clear cut decision to make.
Then there are the mistakes, everyone make them and it' easy when in a hurry because something else needs doing soon, a meal's on its way or they need to go for a dump - the quickest option may be all that's available. Once something is done, its done and its hard for someone with only moderating privileges to put right. Threads can be moved to a temporary mod/admin only forum for later sorting out then returned but that too takes a lot of work, something they may not have time for.
And then they are constantly being called out, told they are making bad decisions etc and do you expect them to stay & put up with it for nowt?
If a decision appears to have been made that is completely wrong then you are able to ask why the decision went such a way but asking via the admin contact info however the admin are under no obligation to answer you as they too may have a lot to do, many other admin things that take higher priority than answer your questions on a free and public forum that they do not benefit from.
Perhaps some of the posters on here should try and consider what the moderators have to do, put up with and what training they have been given etc.