PHP $_POST can't read values set by jQuery val()

I created a little script which allows users to shadow some input values from other inputs. The JavaScript part is working fine, but now when I'm trying to send the form, it doesn't send it, saying that the field the value is copied to is empty. What's the problem here? Thanks!

$(document).ready(function() 
{
    $(":text").blur(function() 
    {
        var input = $(this);
        var id = input.attr("id");
        var fieldname = "#".concat(id);

        for (i = 0; i < fields.length; i++)
        {
            if (fields[i][4] == id)
            {
                var boxname = "#copy_".concat(fields[i][0]);
                if ($(boxname).prop("checked"))
                {
                    var fieldname2 = "#".concat(fields[i][0]);
                    $(fieldname2).val($(fieldname).val());
                }
            }
        }
    });

    $(":checkbox").change(function()
    {
        var input = $(this);
        var id = input.attr("id");
        id = id.substring(id.indexOf("_") + 1, id.length);
        var fieldname = "#".concat(id);

        var checked = input.prop("checked");
        $(fieldname).prop("disabled", checked);

        var field = FindField(0, fields[FindField(0, id)][4]);

        if (checked)
        {
            var fieldname2 = "#".concat(fields[field][0]);
            $(fieldname).val($(fieldname2).val());
        }
    });
});

function FindField(index, value)
{
    for (i = 0; i < fields.length; i++)
    {
        if (fields[i][index] == value) return i;    
    }
    return -1;
}

The form (without some irrelevant stuff):

<form method="post" action="sendform.php">
    <table>

        <?php
            foreach ($fields as $field)
            {
                if ($field[0] == "divider") echo('<tr><td colspan="2"><hr /></td></tr>');
                else 
                {   
                    switch ($field[2])
                    {
                        case FIELD_TEXT:
                        {
                            echo('<tr><td><label for="' . $field[0] . '">' . $field[1] . ': </label></td><td>');

                            if ($field[4] != "") 
                            {
                                foreach ($fields as $field2)
                                {
                                    if ($field2[0] == $field[4])
                                    {
                                        echo('<input type="checkbox" id="copy_' . $field[0] . '" title="Kopeeri väljalt &quot;' . $field2[1] . '&quot;" />');
                                    }
                                }
                            }

                            echo('<input type="text" id="' . $field[0] . '" name="' . $field[0] . '" placeholder="' . $field[1] . '" /></td></tr>');

                            break;
                        }
                    }
                }
            }
        ?>

        <tr><td colspan="2"><hr /></td></tr>
        <tr>
            <td colspan="2" style="text-align: center;">
                <input type="submit" value="Saada avaldus" name="saadaavaldus" />
            </td>
        </tr>

    </table>
</form>

A sample of the fields array:

var fields = <?php echo json_encode($fields); ?>

$fields = array
(
    //      ID/name         label/placeholder                       type            regex                                                          field copied from
    array(  "saatjanimi",   "Teie täisnimi",                        FIELD_TEXT,     "^[-,.'\s\pL]*\pL[-,.'\s\pL]\s+[-,.'\s\pL]*\pL[-,.'\s\pL]*$",   ""),
    array(  "meiliaadress", "Teie meiliaadress",                    FIELD_TEXT,     "^([\pL-.\d]+)@([\pL-.\d]+)((\.(\pL){2,63})+)$",                ""),
    array(  "isanimi",      "Lapse isa täisnimi",                   FIELD_TEXT,     "^[-,.'\s\pL]*\pL[-,.'\s\pL]\s+[-,.'\s\pL]*\pL[-,.'\s\pL]*$",   "saatjanimi")
);