Why wouldn't you? You're all the way there and pointing a sensor at the Earth isn't a big deal. You know you can do more than one thing at a time, right?
Climate science is probably one of our bigger planetary issues and perhaps the biggest issue facing humanity right now. You know Earth is a planet right? That's part of planetary science, that's why you explore space in the first place. It doesn't get an exception for purely political reasons. Studying the Earth is no different than studying Mars or Jupiter.
Its incredible the hoops people will jump through to justify Trump's absurd climate conspiracy theories and asinine policies in regards to science.
For the same reason that these aren't on the Moon:
https://en.wikipedia.org/wiki/List_of_Earth_observation_sate...
There is plenty of peculiar-looking down-voting on HN. I am seriously beginning to wonder if that down arrow serves any useful purpose.