Where have you heard that it is illegal to do so? It isn’t illegal and shares are offered at a discount to their market value to encourage the shareholders to purchase the shares. It therefore ensures that the required amount of funds are raised by the company to invest in projects to increase shareholder wealth.
It was said in the LW paper that the shares couldn’t be sold below their par value (ie they could not be sold at a discount). I believe it was even mentioned in the FA paper.
You are confusing market value and par value. When the shares are issues in a rights issue it is done at a discount to their market value, not their par value.