I think it likely that this is, in part, due to attempts at making it easier for some students to advance, which would have caused them to study far more than more gifted ones, thus alienating them from their "peers". In other words, possibly hurting their feelings, a known horror to be avoided at all costs (including the education level of those who could do better).
However, from the sound of things, you may be very good at math (I myself am probably average), and in the USSR they likely recognized that and focused on it in their instruction of you. This has itís advantages (improving an area that is already a strength), but if they reduced your instruction in other areas it would limit your overall education to an extent.
Now, as to the beating of kids if they did poorly, that cannot be done in the USA at this time (at least not without repercussions for the parents). And further, I personally think there are better ways, which avoid potential mental/physical harm to a child.