And policy design -- not to mention civ design -- are also adjusted periodically. But they're basically balanced for standard. So if you want to balance different-sized maps without upsetting the entire apple cart, starting with adjustments to the % variables makes a lot of sense... hence, the map-size scaling thread.
I wouldn't call it a problem, but I do think it contributes to less-competitive games. Speaking broadly, it benefits the cautious, lower-ranked civ by providing protection, and the already successful one with additional benefits. Does the AI need more protection methods, especially now that they can benefit from the new DP's? I don't see the readon for the overlap, and would prefer vassalage to be limited to more extreme cases of imbalance (and of course peace treaties).