You have missed the bit about the Sustainability Cap
In advance of each Season each Club is required to submit to the Rugby Football League an Operating Budget for the Season (in a standard form). This is scrutinised, where appropriate supporting evidence required and then signed off. This agreed Budget determines what a Club can spend on players to either breakeven or make a small permitted loss – this is called the Club Sustainability Cap.
Under the Salary Cap Regulations each Club is restricted to spending on players the lower of the finite Cap and its Club Sustainability Cap.
From here:
http://www.rugby-league.com/financial_sustainability
SL clubs have to follow this according to section 3.1.1. For the Championship just about the whole of section 1 of their version of the salary cap rules are about preventing clubs trading beyond their means and protecting them from insolvency. Section 3.1.1 for the Championship SC rules also starts by saying a club must ensure its aggregate liability must not exceed the Club Sustainability Cap. Same for the League 1 version of the SC rules.
What does this mean in practice? Well for smaller clubs the only way they can pay up to the cap limit is if they get a huge sponsorship. They aren't going to be making enough income from gates receipts and other items the RFL tots up to to give them their Operating Budget.
Whether Leigh spent up to the cap to achieve promotion we don't know without looking at their accounts. They most likely outspent every other side in the Championship though. So much for a level playing field enforced by the SC!
I bet HKR have done the same this season. So P&R is a bit of a sham. The relegated SL sides are most likely to be able to retain their income and keep a squad of players likely to compete for promotion the season after. The only way this won't happen is if another team gets a huge sponsorship to allow them to compete.
This flies in the face of the stated aim of the salary cap (as in the SL SC rules) which is to ensure success is based on on-field excellence and not financial clout and also means should they lose that sponsorship if their sugar daddy walks away then they will be in breach of the rules.
A couple of interesting things I noticed in looking into this.
1. The expense of running an academy side reduces the Operating Budget so lowers how much a club can spend on the cap.
2. If a club makes a loss outside that permitted, that must be made good the next year or the Sustainability Cap is reduced by that amount.
I am not sure if these two rules apply to SL clubs as they are in the League 1 version of the rules but they are the only version of the SC rules that define what makes up the Operating Budget within the actual SC rules themselves. I have not had time to hunt down any other rules about it that may apply differently to SL or Championship sides on the RFL web site.