First off, it’s in violation of international space law…The Treaty on Principles Governing the Activities of States in the Exploration and use of Outer Space including the Moon and Other Celestial Bodies, known as the Outer Space Treaty.
January 1967 the United State, United Kingdom and the Soviet Union opened the treaty for signature in their respective countries; it was entered into force October 1967. There were 107 parties to the treaty as of 2017 with 23 more signing on but not completing the ratification process. In a nutshell, the treaty calls for space exploration for scientific purposes only and absolutely forbids military usage or weaponization.
Second, it’s an end run around Congress whose job it is to make such decisions and appropriations. Former Navy pilot and Oklahoma Congressman Jim Bridenstine introduced and lobbied for his American Space Renaissance Act in the spring of 2016. The bill was introduced to Congress and was referred to 15 Congressional committees and subcommittees. There were no other actions on the bill by Congress.
Third, this politicizes the one agency heretofore led by scientists. Bridenstine was an early Trump supporter. One of the first beach head teams after Trump’s inauguration was NASA (SCOTUS Justice Kennedy’s son Greg was senior financial advisor). The team of “advisors” was paid more than NASA staffers and had little to no scientific experience. Then Bridenstine was named NASA’s director. It was the first time a politician with no technical experience, not a scientist, was named director. But despite skepticism (most notably from Marco Rubio) he was confirmed, despite past controversial stands on LGBT rights and climate change.
Fourth, NASA has a budget of $18.5 billion, 18,000 federal workers, and over 60,000 contract employees. Bridenstine, according to the Project on Government Oversight, mismanaged the small non nonprofit Tulsa Air and Space Museum causing hefty financial losses. There’s also question of self-dealing.