Unfortunately, the text-indent property is inherited, which means that the assumption that word makes that not setting a text-indent on an individual element is equivalent to text-indent:0 is not actually true. And if calibre enforced that, then it could potentially break the layout in a document produced by someone who, correctly (unlike Word), assumes that not setting text-indent does not imply that text-indent is zero.
What I'm saying is that adding text-indent:0 to an elements style changes that style in a potentially destructive fashion, even in the presence of text-align:center. That change may well be desired in a majority of cases, but it should not be applied automatically. And as I said before I'm not willing to create a new option for this.