I think the Western Region are well within their rights to do this, in fact I would say that it is sensible management of train loadings. The conditions of carriage state:
"Restrictions apply to the use of some tickets (including those bought with a Railcard) such
as the dates, days, and times when you can use them, and the trains in which they can
be used. These restrictions are set out in the notices and other publications of the Train
Companies whose trains you are entitled to use. If a restriction applies and the ticket you
are using is not valid for the train you are travelling in, then:
(a) you will be liable to pay an excess fare (the difference between the price
paid for the ticket you hold and the price of the lowest priced ticket
available for immediate travel that would have entitled you to travel in that
train for the journey shown on the ticket); or
(b) in the case of some types of discounted tickets (as indicated in the notices and
publications) the relevant parts of Condition 2 or 4 will apply."
http://www.nationalrail.co.uk/system/galleries/download/misc/NRCOC.pdf
As stated in the above posts, this is clearly stated in Great Western's notices, the restrictions are clearly linked from the page on the National Rail website promoting groupsave and internet booking engines are correctly programmed.