I got an email from my credit union about something called Regulation D. Does anyone know why is the federal government interested in limiting the number of transactions I can make on my savings account and why should it be any of their business?

Federal Regulation D, also known as Reg D, places limits on savings accounts at all Financial Institutions.

Which transactions are limited under Reg D?
The following types of transactions in savings or money market accounts are limited to six (6) per month:

Overdraft transfers from your savings account to your checking account
Online transfers from savings accounts
Transfers processed over the phone
Automatic or preauthorized transfers, such as bill payments or other recurring transfers
Transfers made by check or debit card