Track athletes have been paid for years, usually in the form of money paid to a trust fund. Money could be withdrawn from the trust fund to cover necessary expenditure like training, food or fast Italian cars. Carl Lewis raised in his autobiography the case of an athlete who retired, was given full control of his trust fund, depleted it and then returned to the sports arena. No sign of the IAAF dropping that extra 'A' though. There are probably 3 reasons for the amateur requirements being dropped, and they're all pretty reasonable when you think about it 1.
Amateur status is difficult to define, let alone enforce.