Why blank line break collapse?

When the user press in the editor shift+return
it generates a <BR />,

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fckconfig.js:

FCKConfig.EnterMode = 'p' ;
FCKConfig.ShiftEnterMode = 'br' ;
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but if the user POST the data to the server
the editor collapse all breaks that haven't
a &nbsp; or another character.
While when the user press only the RETURN key
the editor put a nbsp by default, but when posted
to the server the breaks inserted (<p>&nbsp;</p>),
collapses too, even if the tags are present in source.

When the user reload the editor with its content
the breaks are not collpsed and are visible again.

How to avoid that breaks are collapsed in the editor??