zero weights in weighted.mean

I wonder about the last sentence in the Details section of the 
documentation of 'weighted.mean':

"However, zero weights _are_ handled specially and the corresponding ?x? 
values are omitted from the sum."

The return value of weighted.mean.default is

sum((x * w)[w != 0])/sum(w)

and indeed, it looks as if zero weights are getting special treatment, 
but what is wrong with the alternative (equivalent?) expression

sum(x * w) / sum(w)?

Is it a good idea to remove zeros from a vector before applying 'sum' to 
it? I don't think so. Anyway, the sentence in the documentation seems to 
be uncalled for.

G,