A lot of farming methods are 'dirty' and most people are ignorant about exactly how their food is produced - and are happy to stay that way because they know that they are unlikely to be happy about some of the truth. Often I have been told by someone that they just couldn't give up eating meet and would rather not know how it got to the supermarket. I rarely discuss being veggie as I know it can make some people uncomfortable. After all, it was my personal choice, but I don't believe it's my duty to convert anyone else.
As a long time vegetarian with a vegan brother, I can tell you that two reason for our choices are farming methods and the lack of control we have over how food is produced. Food production and animal welfare are subjects most vegetarians and vegans are clued up on. I have yet to meet a vegan who is well aware of where her food from. In fact, many grow their own to make sure they do - think about the waiting lists for allotments. Raising animals is not normally a choice for them for obvious reasons. I've spent many happy times on two farms(one dairy and one sheep), but am well aware that not all are as well run.
A vegan as shadow DEFRA government minister (or whatever the name is now) is an interesting choice, but it shouldn't prevent her doing her job. She's probably better informed than most.