A more likely explanation is:
1) They don't want to help competitors cherry-pick the most profitable AdSense sites by offering higher rates;
2) Their compensation formula isn't as simple as a straight percentage split;
3) They want the ability to adjust the compensation formula over time--e.g., by paying more for certain levels of revenue of profitability;
4) They don't want to deal with constant complaints from publishers who can't understand why Johns-big-and-highly-profitable-site.com is getting a higher payout than Jims-small-and-barely-profitable-site.com.
If you're really offended by Google's secrecy, there's an easy solution: Pull the AdSense code from your pages. The Google TOS allow a publisher to suspend participation or withdraw from the network at any time.