A number of reasons
Pubs are usually a more controlled environment than an out door festival.
Pubs are run by professional (mostly) publicans not brewers
Outdoor events will attract a much more diverse crowd including families, the risk just multiplys
This is an exercise in arse covering and cost control. One can't admit that to the public so one blames H&S or the Fascists as we are referred to. I would like to see the risk assessment that recommended the use of plastic, who performed it and their credentials.