The problem with practices such as this is that they exist in the limbo between religion and tradition to such an extent that it's difficult to tell which one is which... well, it's not that hard actually...
An example of this is the tradition of Baptisement. This is an entirely invented religious ceremony that is performed to rid the infant child of original sin (!). It was performed by John The Baptist upon Jesus. Since then it has been a religious requirement for all Catholic newborns to be Baptised (or Christened – the terms appear to be interchangeable).
Now, if religious practices and cultures don't interact and mix together then there would be no need nor any desire for non-religious couples to 'Christen' their infant child. But they do. In their droves. And I believe it's very lucrative.
This religious requirement has spilled into mainstream culture and as such, non-religious couples 'Christen' their babies because 'it's the done thing'. A religious requirement has now become 'cultural'.
The point being, Cowtipper, is that despite your claim that, “... this isn't a religious thing.”, I think you'll find that it is.