.disable() does not add a disabled attribute to the input element

$select_region = $('#form_region').selectize();
select_region = $select_region[0].selectize;
select_region.disable();

gives me:

<select id="form_region" name="form[region]" required="required" class="selectized" placeholder="Region" tabindex="-1" disabled="" style="display: none;">
    <option value="" selected="selected"></option>
</select>
<div class="selectize-control single">
    <div class="selectize-input items required not-full has-options invalid disabled locked">
        <input type="text" autocomplete="off" tabindex="" placeholder="Choose a region" required="" style="width: 173px; opacity: 1; position: relative; left: 0px;">
    </div>
    <div class="selectize-dropdown single" style="display: none; width: 301px; top: 36px; left: 0px; visibility: visible;">
        <div class="selectize-dropdown-content">
            <div data-value="1" data-selectable="" class="option">first</div>
        </div>
    </div>
</div>

The problem here is that because the input is not disabled, the browser will prevent the user to send the form. I don't think that it's the expected behavior. Can you please fix this?