Mobiles are big business, and they generate huge profits on some higher-end plans. It doesn't surprise me that they can afford to give away a £100 (cost price!) mini-moto with the phones. Normally the cost of the handset is discounted by this profit margin to make it more attractive.
You'll probably find its for a £35 per month contract for 18 months, without a free phone. Work out the cost for the period of the contract and then see if its still a good deal!
You can always pick them up cheap on ebay, eg:
http://cgi.ebay.co.uk/LATEST-MODEL-2...QQcmdZViewItem