It's similar to pork -- congress can write: spend $X on whatever; or they can write: spend $1 on project A, $2 on project B, etc with a final total of $X. If congress writes the first, then the executive branch will make the decisions about the specifics of how $X will be spent.
If congress does not want the executive branch to decide, it's up to Congress to put the details into the law, and restrain the authority of the agencies. Federal rules can never override a law passed by congress.
http://press.uchicago.edu/ucp/books/book/chicago/I/bo1743668...
See: https://pdfs.semanticscholar.org/3d45/15c4f13933f50394a0a220...
> The post-New Deal administrative state is unconstitutional,* and its validation by the legal system amounts to nothing less than a bloodless constitutional revolution.
Regulatory agencies such as FDA and EPA exist because it actually takes an agency to do a competent job regulating drugs and medical devices and investigating the toxicity of effluent. They were created by Congress, but under the Executive branch.
https://www.federalregister.gov/uploads/2011/01/the_rulemaki...