shorts?

I was hoping to connect with the Upaya Zen Center in Santa Fe but I only brought shorts because American Airlines charges $25 for a bag to check in and I had tight space in my backpack. A pair of jeans would take up a lot of room, and I don't expect to wear them. Upaya's website says, "Please do not wear shorts of sleeveless tops." I find myself asking, "what does that have to do with meditation?" And they want you to wear dark colors. I'm so curious what the thinking is behind this rule, and how these traditions cropped up. If anyone knows who reads this, please comment.