In a word: ahimsa.
Ahimsa is the principle of non-violence towards all living things. Adopting a vegan diet and lifestyle is one of the strongest statements of ahimsa a person can make, wherein one refuses to participate in any action which is connected to violence towards a living being.
Not all yogis are vegan. However, those that are not tend to gloss over the teaching of ahimsa, or insist (incorrectly) that adopting a vegan diet would be himsa (harmful) towards oneself. One prominent school of yoga that puts forth veganism as the true statement of ahimsa is the Jivamukti school. Many others gloss over or do not understand the immense suffering caused by the dairy industry, and so recommend a vegetarian diet that includes milk.
Ron Paul is apparently not a vegan. However, he understands the principle of ahimsa, at least as it applies to politics:
One rule you have to follow: and that is, in order to make the world a better place, you have to reject the notion that you use violence and force to do it. You have to do it through persuasion. If we want to change the world, if we want to change the Middle East, if we want to change Africa, you do it by spreading ideas and through persuasion. But the most important thing we do is do our job here, set an example, so others would want to emulate us, and that should be our [goal].
– Ron Paul (from this video)
There’s a reason for all this. It’s not just because somebody handed down the rule of ahimsa from on high. The reason to follow ahimsa is because it works. In the realm of politics, if you try to make the world a better place by force and occupying other countries, you get the “blowback” effect, namely, you create resentment and anger towards the occupying country. In the realm of inner peace and meditation practice, you cannot find peace on the inside without living peacefully on the outside, i.e. a vegan lifestyle.