I didn't know this either. I guess that's why many projects have a fake $1 tier with just "the pleasure of knowing you contributed" or something like that as the "reward" for that tier. To let people know that that's still an option.
I agree, I would have signed up for a $1 tier or perhaps $2 but $15 is far too high for the minimum tier. I support the Zig author for $1/mo and hope others would do the same for my open source projects in the future.