English Geek's advice is excellent, however, I'd like to provide a more specific answer:
Unlike "to talk about," "to discuss" is transitive, i.e., it takes a direct object, without the preposition "about." There's no reason – that's just how English is. Actually, English Geek makes more sense than English grammar....

Phil, I don't think there is 'no reason'.

There are transitive verbs, like
"I love you"
and here we think about an action directed at an object.

There are intransitive verbs, like
"I dance with you"
And here we focus on two people dancing together.

And there is occasional transitive usage:
"dance me", as in Cohen's song.
The meaning is different. And there is a certain difference between "I met you", "I met with you" and "we met".

So it happens, speakers need some verbs to be transitive sometimes.

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ANother peossible answer to 'why' lies in the etymology of the word.

You do not need "about" since you want to discuss the "issue". It is implied that anything that the "issue" entails will be discussed. 

You can say "let's talk about the issue" or "let's discuss the issue".

The differences between talk and discuss are slight, but when you talk, you "talk about things", when you discuss, you "discuss things". It is just the form.

*perhaps you should consider rewriting your question. Is it supposed to be a philosophical question or a grammer question?

I prefer to interpret the question diferently. It is wrong because nobody should have to worry about evil clowns. What was Stephen King thinking of?

It's simple, Ritam.

Even the Right Answer to the Wrong Question Is Still the Wrong Answer.