That's plugins, and extensions in general. Enabled or not, the code should be checking if it has the data it needs to work or gracefully abort (doing nothing or showing a message if appropriate).

I'am alreay checking in my component that public key and private key are not empty before to use captcha plugin

I'm not sure what the issue is.
If I enable the plugin without giving the keys, I get a clear error message on the pages where it is supposed to load (eg contact form).

In the plugins list if you filter by captcha and you don't have entered private/public keys, you can still enable with the button like on the screenshot without editing the plugin :

Yep, as said you can enable it. There is no way to prevent this the way Joomla works.
But that's no real issue. The plugin itself checks during runtime if it can show the captcha and will show an error when it can't.
Since the admin who enables the captcha hopefully also tries the new feature he is going to add, he will see the errror for sure and can take the needed actions.

Or did something break on your site after you enabled the plugin?

The same is true with many other plugins

On 2 June 2015 at 15:21, Thomas Hunziker notifications@github.com wrote:

Yep, as said you can enable it. There is no way to prevent this the way
Joomla works.
But that's no real issue. The plugin itself checks during runtime if it
can show the captcha and will show an error when it can't.
Since the admin who enables the captcha hopefully also tries the new
feature he is going to add, he will see the errror for sure and can take
the needed actions.

Or did something break on your site after you enabled the plugin?

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Can this be closed as "expected behaviour" ?

Closed as expected behaviour