SELECT ... WHERE column LIKE parameter||'%'

[Heini Nolsøe]

I have a database containing alot of procedures with this kind of coding logic:
BigTable
column1 varchar2(40);
Procedure SomeProcedure (in_parameter varchar2)
SELECT column1
FROM BigTable
WHERE column1 LIKE in_parameter||'%';
END;
The procedure is called like this:

SomeProcedure('abc');
Although this coding logic selects the correct rows, there is a big problem - it does not use the index on BigTable.
But if you do perform at little test and change the procedure like this:
Procedure SomeProcedure (in_parameter varchar2)
SELECT column1
FROM BigTable
WHERE column1 LIKE 'abc'||'%';
END;

Then it uses the index. But ofcourse now there is not the desired flexibility of using a parameter.
QUESTIONS:
1. Why does Oracle not use the index when a parameter is used in the LIKE-statement?