It's a historical restriction that dates back to the very beginning of pension saving when it started in 1917 under the Income Tax Act. At that time, you could only have employer contributions to a pension plan. That changed in 1946, I think it was, when employee contributions were permitted. But those restrictions have carried through, and the Income Tax Act essentially says that you can only have pension contributions in respect to your income from an office or employment, which means employment income, and your employer has to pay for those defined pension benefits.
Under the Income Tax Act, the amount of money that you can put away for retirement is much greater under a defined benefit arrangement than it is under an RRSP. So there's this real inequality between the two kinds of plans. This is one of the reasons why the public sector plans are better: because they're all defined benefit plans.
So what I'm really suggesting here is that we disaggregate the employment relationship from pension savings, so that anybody can contribute taxable income to any pension arrangement within an aggregate lifetime tax limit. Essentially, eliminate the distinction between defined benefit and defined contribution plans, and just give everybody the same limit. Allow anybody to contribute up to that limit in any arrangement they so desire, whether it's be pension plan or an RRSP.