CAPTCHA - part 1 : "Are you human or ....?" (Introduction)

Why we must use this item for security?

<Form method='get' action='logon.php'>
Put your name please <Input type='text' name='name'><br>
<Input type='submit' name='submit' value='submit'>
</Form>

This form will collect information from user. This information will be stored at table in database. When somebody submit this form, you can see this url in address bar:

Now, what a weakness about this way?

Imagine, there is somebody write this code:

<?
for($i=0;$i<=1000;$i++){
$lines = file('http://www.mysite.com/logon.php?name=Just+Kidding');
}
?>

And then he upload in anything server. Execute that code. Without open legal form, he can fulfill that information. Not only one information, a thousand. How about he rewrite become:

<?
for($i=0;$i<=1000000;$i++){

}
?>