What causes a checkbox value to always be 'on'?

Thanks for your previous help with identifying a jQuery script as blocking the form value. I've tested with various configurations in MySQLi Insert Record SB, but the value returned in the database is always a "1" even when the checkbox is unchecked. I've set the Column Binding to

<?php echo ((isset($POST["emailReceive"]))?"1":"0"; ?>

with Submit as set to Checkbox 1,0, and Blank value set to 0 to insert either a 1 (checked) or 0 (unchecked) into the MySQL database field.

The jQuery script merely puts all the form values into variables, so as long as the element is included the script works as expected. The input field looks like this:

<input name="emailReceive" class="checkbox" type="checkbox" checked="checked" value="1" id="emailReceive" tabindex="6" style="width: 1em; height: 1em;" />

I'm able to echo the $_POST["emailReceive"] to the result page but it is always on, even when the checkbox is unchecked. I've looked at tutorials and other posts, but can't find pointers on how to fix this.

I've attached the related PHP, and .JS files. the menu01f.php is an include file where the form is located.
Thanks so much!