The definition of line training is that it is done on line - with passengers and/or freight on board. Can you imagine how much it would cost to provide line training on empty aircraft? It would make the costs of an Oxford ATPL pale in comparison! Think in terms of £5,000 - £10,000 per flying hour...
100 hours (about 40 sectors) is enough to learn what you need to know to be moderately competent - and more than enough for any training organisation to judge whether or not you should continue. Any more than that and I think those who claim this is cheap labour would have a point! 500 hours, even if it were morally acceptable, would be prohibitive in cost for the student. In any case, it would be time and money wasted; any decent airline will wish to give new employees training in their own SOPs and ways of doing things, which will be done in their own line training programme - albeit possibly a truncated one for those who have done an SSTR.
Whether you like it or not, SSTRs are a feature of the speculative flying training scene, and are actually no more difficult to justify than IR or MCC training. My concern is that students get value for money in that training, and that the training itself is valued by the industry. There are doubts on both those issues with some schemes, but that is no reason to rubbish them all.
Scroggs