In the example problem, why are the 8 minutes being subtracted from 6: 15pm since the longitude is 73 degrees west?
it states that west sunsets are experienced "later" (we add time) and east sunsets are "earlier" (we subtract time).

Good question. What they mean to say is that locations that are "more East" are earlier, while locations that are "more West" are later.

The starting city was at a longitude of 75* West. Notice that 73* West is more to the East than the starting city (in other words, it is "less West"), while 78* West is more to the west than the starting city.

This is why we subtract 8 minutes going to 73* West from 75* West.

They immigrated for religious freedom