Well the short answer to this is "Yes they can" they just do it weather it is morally or legally correct is another matter!
I'm afraid I am back to my old friend Universal Utilities (t/a Unicom) for this one but I'm sure there are others at it. In this instance Unicom have come up with a new product called "Fraud Defender" which thy have applied to everyone's telecom account, and although it is free for a trial period it will become chargeable after the trial expires.
From what I have been able to attain it will cost you about £1.95/Month after your trial has expired. Not a lot I hear you say, true I reply but it's the principle that they are adding services to your account without your consent or knowledge and then charging you for the privilege.
Ok so it's £1.95 this month what comes next £4.99/Month for the TeleSales Tea Fund or £10.99 for the CEO's holiday.
What is "Fraud Defender" well in reality as far as I can see it is no more than access to a SQL Database or the like running a script looking for abnormal trends in your call patterns. So it is data they are already collecting just with a different script interrogating it, so if they have 100,000 customers as they report at £1.95/Month that equates to a pre-tax profit of some £2.3 Million, not bad for old rope.
My biggest fear of this tactic is that if you let the trial period expire, when you notice the item on your invoice at a cost to you, if at this stage you try to have the product removed you will get a response like "Well you had x number of days to evaluate and cancel this product, so I'm afraid that it is now locked in for your current contract period". Call me cynical but I have seen it all before, these companies are sharks and will do anything to get every last penny out of you.
If you find it appears on your Invoice or On-Line account and you feel it's not for you then ring up and cancel it, albeit the cancellation isn't easy.