Pretty much every question about WHY we do something can be answered by: "Because the IRS says so."
We are a 501(c)7 - a Tax Exempt Social Group. We're allowed to pool our funds for member events, to sell items to members, and take donations from members.
(While there's some leeway in the tax code for a percentage of income from the general public, it's a real pain to calculate.
Let's not make the Treasurer Wah angry. You won't like him when he's angry.)