Can you explain why it is nashi, not nashi pear and why the
names for the fruit are often interchanged?

Nashi is actually the Japanese word for pear. Nashi are also
occasionally referred to as Asian pear, apple-pear or Oriental
pear because they are native to northern Asia. They have taken
many hundreds, if not thousands of years, to evolve from their
very distant relatives, the European pears that we are so familiar
with, such as the Williams, Packhams, Triumph and Beurre Bosc varieties. Unrelated to apples, the long isolation of the nashi in the East has lead to the development of its own unique crisp, delicately sweet and juicy texture, with the fruit harvested ready to eat.
This contrasts with European pears which are soft and juicy
when ripe but are harvested much earlier, meaning most of us
have to wait to enjoy their delights!

For the botanically minded, nashi belong to the species Pyrus pyrifolia while European pears belong to the species Pyrus communis