Bug: Can’t assign to hline relative reference

[Dante Catalfamo]

Hello,

I'm trying to create a table where the third column between two hlines is set 
by a formula referencing the second column.

Here is an example:

| a     |     b |      c |
|-------+-------+--------|
| row 1 | 12.00 | 144.00 |
| row 2 |       |  23.00 |
|-------+-------+--------|
| sum   | 12.00 | 167.00 |
#+TBLFM: @I$3..II$3=if($2 != 0, 12*$2, 
$3);%.2f::@4$2=vsum(@I..II);%.2f::@4$3=vsum(@I..II);%.2f

When I try to calculate this table however, I get the error "Can’t assign to 
hline relative reference".

Why is this now allowed? This seems like a fairly mundane use of a table to me. 
It is explicitly disabled in the source, but without explanation, and searching 
the mailing list doesn't turn up many answers.

Is there a recommended way to handle a situation like this?

Could this feature be fixed?

Thank you,
Dante