How does a band like DMB become a pot band?

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This is a curiosity since I don't really like these bands or the subject matter. So how does a band like the Dave Matthews Band become known as a Pot band? The are obviously other rock concerts where people go and smoke, but DMB is notorious for being a weed friendly band. It is just like Grateful Dead was and Phish is. Do they follow a particular political or activist group that supports this? Is there a signal they give that says they are like this? Is it a musical style? A catch phrase or behavior used? I am just curious how this band gets labled as such where say a heavy metal group is not, even though there may be people in that audience lighting up just the same.