Be careful not to mix up two different things here. This policy has nothing to do with whether an internal transfer can be offered the same salary as an external hire, even if it might have that side effect from time to time. Policies like this are designed to prevent employees from gaming the system to artificially1 increase their salary and bypass normal pay-raise mechanisms.
Here's an example. When the 2008 recession hit, a company I used to work for halted all pay raises for a couple of years in order to avoid layoffs. We had quite a few employees figure out that they could leave the company and our cross-town rival would hire them for about 7% above what they were making before. Because of the recession, it would be a year or so before their team could get clearance to hire a replacement. Once they saw a job posting for their old position, they would apply and get re-hired (since they were the perfect replacement for themselves) - again at a salary about 7% above what they were making. In a little more than a year, they would end up in the same job they started in, but with a salary almost 15% higher than where they started.
When my company saw that this "technique" was being exploited by dozens of people for the sole purpose of bypassing the company's pay-raise system, they instituted a new policy: When re-hiring a former employee who has been away from the company for less than X amount of time, you can offer them no more than Y% above what they were making when they left. This made the game a lot more trouble than it was worth, and the practice soon halted.
The OP's case deals with internal transfers and not re-hires, but it's really the same game. The pay raises are typically smaller for internal transfers than for poaching a competitor's employee, but you can still exploit that system in the same way to job-hop and inflate your salary. Since you're staying within the same company, it looks less fishy on your resume as well.
Always remember, though, that rules like this are designed to discourage people from trying to game the system. They're not set in stone. If there's an internal candidate that would be perfect for your team and there's a justifiable reason to pay them that much, hiring managers can typically escalate the issue to HR or a VP and get an exception made. They might add a precautionary clause or two to their contract, but having management vouch for you can go a long way.
1 I use the term "artificial" here because the pay raises did not go through the normal raise process and were not based on merit. Employees were essentially engaging in price-gouging to inflate their own salary. They would repeatedly emphasize that the short-term cost of finding another replacement and training them to the same level of proficiency would cost more than that 15% differential. The company was very focused on short-term finances due to the economic situation, so the company would suck it up and re-hire them at the inflated salary, even when it was well above market rates. In the long term, that employee would cost the company considerably more than a comparable employee. Pay couldn't be reduced if the employee was performing adequately, and "because you're overpaid" wasn't considered a valid reason for firing somebody. Instead, these artificially-inflated salaries remained, reducing the funds available to use for giving everyone else raises.