Actually veganism is much older than the word 'vegan', which was coined in the 1940s. There are many eastern religions, such as Buddhism and Jainism, that encourage various forms of veganism (not all of them necessarily healthy diets) in their monastic orders.

However, I think you are fairly liberally type casting vegans as single issue people. I would argue most vegans probably know about where their food comes from than you, as well as the affects that other parts of their lifestyle have on the environment.

From my own personal experience, I know there is exactly one vegan organic standard (which is the VON's "Stockfree" certification a sub-certification of British Soil association's UK-5 standard). That means if I eat organic produce living in the US a large amount of the fertilizer used is probably of animal origin. However, I'm between the rock and the hard place because I either choose harm the environment (and animals in other ways) or accept that some of the organic fertilizer will be manure. I didn't set up the system it isn't my fault. However I will choose to opt-out where-ever I sensibly can.

In my experience people become vegan for a variety of reasons, but most of those reasons are because they are conscientious, caring people. Like all groups there are a vocal minority who wish to harass other who disagree with them, but I don't like be tarred with that brush any more than you like being accused of murder.