I am having this problem on all my sites drupal7, except one.
I see no differences in settings, I read and checked the IMCE-profile as described on http://drupal.org/node/341153. No avail.
Only difference I see is the translations: on the one where the button shows up to all users, the translation of IMCE/TinyMCE is not 100%: some button-names are translated, some are not; while on my other sites, translation is complete. (Dutch)
I do not know a way to remove translation for one module only, because removing all translations is no option. I could remove some sentences, if I would know which ones are likely to be causing this issue?