They do not deliberately send you 5 text messages. Your phone tries to send your text but Safaricom returns an error (because you don't have cash). So your phone retries sending the message which again fails and the cycle continues. Meanwhile for each failed attempt, Safaricom sends you an sms about your account status.

Date: Fri, 21 Sep 2012 17:31:41 +0300
From: themburu@gmail.com
To: skunkworks@lists.my.co.ke
Subject: Re: [Skunkworks] Mpesa question: airtime to mpesa

If it is a product as you want to put it then why would Safaricom send you 5 sms's telling you you have insufficient credits to send one message??