This was how David Denison (Manchester), in a paper jointly presented at the Helsinki Corpus Festival with Marianne Hundt (Zurich), described the American phobia for which in non-restrictive relative clauses.
American users are advised to “use that before a restrictive clause and which before everything else”, but, as Denison pointed out, British usage is different, allowing both which and that in restrictive relative clauses (for instance, “Gems that/which sparkle often elicit forgiveness” – the example is one provided by Grammar Girl).
Deborah Cameron, in her book Verbal Hygiene (1995), discusses what she calls an absurd example of the rigorous requirements of style guides in this matter. A case of conflict arose over the choice between which and that in restrictive/non-restrictive relative clauses in a collection of academic papers from both British and American authors. The collection was copy-edited for the American market according to the guidelines in the Chicago Manual of Style, which prescribes that in restrictive and which in non-restrictive relative clauses.
The British authors revolted, because British English shows a more complex pattern of usage. The solution adopted was to allow each author their national preferences, thus resulting in a lack of consistency that would have gone against the notion of having a style guide to begin with (Cameron 1995:50-51).
Where does the American rule come from? Did it originate from pragmatic reasons, elminating choice in the process? And was the ad hoc solution of the publisher at all satisfactory? Will American usage eventually spread and become current in British English as well?