Category slug variable is not available when category url's are disabled
That's by design, slugs are used for building the url to link to a specific element, in this case, a category.

Edit: seems I was wrong on the above, see comment. Leaving the rest of the answer as using the ID offers a solution. 

You'll have to use the id to refer to the category as value, like so: 

&lt;select name="field-styles" id="field-styles" class="select"&gt;
    &lt;option value=""&gt;Style&lt;/option&gt;
    {% for style in craft.categories.group('productStyles').order('title') %}
    &lt;option value="{{ style.id }}"&gt;{{ style.title }}&lt;/option&gt;
    {% endfor %}
&lt;/select&gt;