A lot of clubs from here and abroad find the away ties hard on them and their fans because of the expense, however they are glad to be in the competition at the time.
I believe getting to the semi final stage gives you as much right as any team to expect a truly neutral venue. Maybe Wire and the RFL were afraid that the fans from Warrington would not travel any further than Widnes.
The RFL like stadiums to look full when on national TV. Maybe they reaslised that the chances of a catalan victory were slim,and therefore wanted a reasonable sized stadium to look quite full and realised that widnes was their best option?