If you bought a pair of trousers that we too long you would get a tailor to shorten them so why no different on bicycle components?
Manufacturers make seat posts in 1 length length, in rare cases 2, in line with the "one size fits all" philosophy, the same applies to frames whcih are typically available in 3 to 5 sizes.
People are different shapess so unles you get a custom made frame, it is a matter of chosing a frame a post that fit best and in the majority of cases the post will be too long.
There is a minmium post inserts, 100 mm or so so any excess post is doing nothing but adding additional weight. The weight is not huge, maybe 1 -2 g per length.
There is nothing wrong in cutting a post to length and some manufactureres e.g. RaceFace encourage this by having marks on the post with a listing of the weight saving. (tried to find a picture of this but failed)
Only down side is when it comes to sell a post - people prefer a post not to have been hacked about.
The WW doubters will quip with "what's the point as you should have a haircut/dump/manicure before a ride" but you can still do that as well as WWing.