Not all of the sales that take place on darknet markets are for personal use; some are bulk orders that will be split up for local resale. Investigating those buyers is well worth the investigative resources.
why is personal use OK but selling for personal use bad?
Personal use is still 'bad', but it's more efficient for police to go after resellers on the streets than all 50 individuals who bought from those resellers.
Why is personal use 'bad' then?
Because the law says it's "bad" and that's why everyone used quotes around "bad".
It's irrelevant if it's "bad". It's illegal, which means that at least in theory, law enforcement is supposed to pursue and prosecute it, since their mandate is to enforce the laws as passed by the legislative body, etc.
Because heroin addicts cause vast destruction to their communities and families.