Oald: The police report exonerated Lewis from all charges of corruption.

Oald: The police report exonerated Lewis from all charges of corruption.
 

In my dictionary

eg The police report exonerated Lewis from all charges of corruption.

The charges must have been raised before being dropped, so it's definite, why not ALL THE CHARGES but simply ALL CHARGES?

Whether the charges were raised before they were dropped is not the determining factor for not using the definite article. It sounds fine the way it appears in the dictionary.

Whether the charges were mentioned previously is unclear from the context. If all the charges were not made known, 'all charges' may be used.
The determiner 'all' allows this construct.