Why do some manufacturers reverse preamp polarity in the first place?

A single gain stage always inverts polarity, would you have the manufacturer add a second gain stage to change it back again?

And do you change the polarity of your speakers for each recording the you play? That would be necessary if the goal is to always be in correct polarity. In fact, many recordings that use spot mikes on different instruments will have the sax in negative polarity while the keyboard is in positive polarity, so I would suggest that you change the polarity of your tweeters, midrange and woofer as called for by the specific recording. 

So at the end of the day, it really does not matter how the speakers are wired. Interesting. 

Several answers above as to why some preamp invert polarity.  
With last years big upgrades I can better than ever hear recording polarity. Lucky for me, my main preamp has a polarity switch easy at hand. Also with dipole speakers it is easy to hear a difference. Since reverse polarity the soundstage is larger and behind the speakers, with correct polarity the soundstage is more in front and between the speakers. Plus with correct polarity vocals are clearer. Bass is slightly more defined.  
There is not only correct and reverse. All sorts of angles.. Also some records multi tracked, some instruments one polarity some the other.. some 90 off due to running through gizmos.. One of the reasons plenty of Rock (in particular) recordings you cannot hear any real polarity... since in one recording it is all over the place.

We've had a polarity switch on our preamps since their inception. Changing speaker cables around is a real pain!