A Fairfax Media investigation published in this newspaper on Friday has revealed McDonald's reached a national agreement with the Shop, Distributive and Allied Employees Association in 2013 (one endorsed by a vote of employees) in which casual employees lost their entitlement to weekend penalty rates, and had their late-night loadings reduced and restricted to the hours of 1am to 5am. To compensate for the loss of penalty rates, full-time employees became eligible for an increase in base pay rates.

McDonald's claims to be not underpaying its workers, arguing the new arrangement provides additional benefits beyond the award, such as annual pay increases and better leave provisions. SDA national secretary Gerard Dwyer, meanwhile, argues that "in the vast majority of cases, this [agreement] leaves the worker better off".

The McDonald's workforce, however, comprises predominantly young female casuals, and under this agreement many of them now earn as little as $10.08 an hour – which is about 42 per cent less than the national minimum wage of $17.29. It's estimated that McDonald's may have saved hundreds of millions of dollars in the years since the deal was concluded three years ago.

Lest anyone think Mr Schlosser's book was a complete hatchet job, his views of McDonald's hiring practices, its policy of entrusting young schoolgoers with responsibility and of providing opportunities for their advancement, was favourable. Indeed, while McJob may have enjoyed currency for a time as a term for unstimulating, low-paid work, McDonald's may properly be regarded as reasonably enlightened and broad minded, far more so than many other employers in the hospitality sector.

Nonetheless, it has been party to an agreement that privileges one classification of worker over another, for which many people will be rightly critical. The ethics and morality of the deal also reflect poorly on the SDA. Then again, the union is no stranger to such deals, having signed an accord with Business South Australia in 2015 to end Saturday and evening penalty rates for retail workers in that state, and to wind back Sunday and public holiday loadings. Like the McDonald's deal, the SDA sold it as being good for permanent employees because it lifted their base pay rates and broadened their entitlements.

If arguments about (imperfect) deals being acceptable because they are for the greater good stand certain scrutiny, they gloss over two pertinent if inconvenient facts: part-time employment has surged during the past decade; and casual employees (many of whom are students) continue to rely heavily on penalties to help them make ends meet.

With no requirement for employers to pay for sick leave or holidays, it's easy to see why casual and part-time employment might be growing faster than permanent employment. This is no bad thing, say business groups, because it affords employers the flexibility they need to survive the lean times and to prosper and expand in the good times. It's evident, however, that the weekend and holiday loadings that made casual work attractive (or at least offset some of its disadvantages) are being eroded. That would be somewhat acceptable if casual pay rates, particularly for juniors, were increased by way of compensation. They are not, which calls into question the ability of the Fair Work Commission to adjudicate adequately on minimum standards of pay and award conditions.

Employers, emboldened by the SDA's preparedness to sell casual workers down the river, are pushing hard for an end to weekend penalty rates. They argue, without any compelling evidence, that their abolition will boost employment and complete Australia's transformation to a dynamic 24/7 economy.

There is still a sizeable segment of the community that believes employees working unsociable hours ought to be adequately compensated. But with unions apparently indifferent to the plight of casuals, and employers ratcheting up their rhetoric, how much longer can this sentiment prevail?