I don't know why you would want to join after the snotty replies to your query, but just in case you are still considering it:

WHAT: Dues policy for new, first time, OPEN full category members.
WHY: To clarify the length of time your membership is valid, according to the
date of your dues payment. This is necessary since AMA membership
works on a calendar year basis starting January 1st, rather than the date of
your dues payment.
HOW: FIRST QUARTER: If you join AMA as a first time, new member from
January 1st through March 31st, you will do so at the full, open rate.
SECOND QUARTER: If you join AMA as a first time, new member from
April 1st through June 30th, you will pay the full, open rate. The
Following year, your renewal notice will reflect a 25% reduction in dues.
THIRD QUARTER: If you join AMA from July 1st through September
30th, you will pay the full, open rate. The Following year, your
renewal notice will reflect a 50% reduction in dues.
FOURTH QUARTER: If you join AMA from October 1st through
December 31st, you will pay the full open rate. You will receive
membership for the remainder of that year, plus membership for the
following year.

Abel