If a journal has optional costs, is it bad form not to pay them? I've noticed that some of the journals I submit to, most of which are top tier, have optional costs.  For instance, there is an optional page cost that I could pay even though my paper is not open access and is under the page limits.  This cost is revealed upon acceptance.

I've always said no to pay it, but I wonder why the costs exist in the first place and if it's bad form to say no?

EDIT: I just realized how arrogant it sounds to say "most of which are top tier" :-P I meant "most of which are reputable journals that would not appear on Beal's list.
I'm not sure which discipline you are referring to, but I have published my articles in social and health science journals. Most journals offer options at a cost (optional Open Access, color reprints, etc.). If you do not want or need these options, there is no reason to feel that it is wrong to decline them. I have never paid these extra costs. Journals that are not strictly open access make their money from subscriptions, not from the authors. The additional costs are mainly to cover optional features that would make your article more costly to publish than other articles without these features. So, no worries!