In my rather limited experience with docs, this is done a lot.
The doc takes a guess at what's wrong and begins treatment. If it works, great - the guess was correct. If it doesn't work, the doc must go to the next step in the process.
In other words, they use treatment as part of their diagnostic toolbox. And this isn't all bad or lazy. It can be the most efficient approach.
For example, if you show up with a urinary tract infection (UTI), most docs will give you antibiotics right away for the most common UTI pathogen, and take a urine sample to culture it. If the treatment works, the doc was right and you're cured. If the treatment doesn't work, they look at the culture to see what they're dealing with. This approach works most of the time, in my experience.