Why is it a bad practice to lock the object we are going to change?

Why is it a bad practice to use lock as in the following code, I'm assuming this is a bad practice based on the answers in this SO question here

private void DoSomethingUseLess()
{
    List<IProduct> otherProductList = new List<IProduct>();
    Parallel.ForEach(myOriginalProductList, product =>
        {
           //Some code here removed for brevity
           //Some more code here :)
            lock (otherProductList)
            {
                otherProductList.Add((IProduct)product.Clone());
            }
        });
}

The answers over there mentions that it is bad practice , but they don't say why

Note: Please ignore the usefulness of the code, this is just for example purpose and i know it is not at all useful