I'm sure it's quite simply: Castle will fix the computers for free (which is fair enough). But the computer has to somehow get to Castle and get back to you. Either you can get it delivered (£25?) or drop it off yourself (£petrol). It's fine taking it to them if you live nearby, but a lot of people are too far away.
So, for us northern types, STD/Simtec offer a similar local service. However, it's not their problem and it's on their time, so you pay them for it. Same as you would if you took it to any dealer or local computer/electronics shop (even though they haven't made any press releases).
John Ballance did refer to Simtec on the Iyonix mailing list, so if they've had to deal with lots of individual queries (perhaps asking for it free) I can understand why this press release has been made.
Whether you think the price is reasonable is another matter.