most of what you say is true.
they can endure such jobs only for 6yrs, beyond which the company that sponsored the visa has to file for a green card (if they do, then the employee can continue suffering for a few more years).If they dont file for a GC, then its a one way ticket back to their home country (or go to Canada with a PR ready).
the 70-30 split, is something I used to fume about, thinking its unfair to the employee etc. But that is how contracting companies work (not just for H1b employers). They pay the employee $.7X, and charge the client $X for every hr worked.
the benefit for the final company is that the headcount of H1b employees on paper remains low, and the cost of H1b kinda gets prorated into the hourly rate. It doesn't make sense to hire an H1b if the company knows for sure that they need her for just one/six months or a year. But a staffing consultancy can certainly justify filing for the H1b, if the same employee can work with 3 clients for 1 year each.
Though this looks like they (the companies filing for H1b visa) are doing something illegal, it is not, as long as a LCA (Labor Condition Application) is filed for the client's location and is approved, its totally kosher with USCIS, DHS and DoL.
Sadly, its hell for those on such H1bs (and I'm glad to never have worked with such consultancies)