While I can see why USAG would want to restrict the difficulty. It does seem a little obsessive to restrict these skills before level 9. They really are quite basic skills. Here in Australia our levels are very similar to those in the USA. So an Aussie level 5 would be similar to a US level 5. But we allow bonus skills in the compulsary levels. The standing back tuck on beam here is allowed from level 5 up for bonus. You dont see many level 5's doing it, but there are quite a few level 6's doing it too. By level 8 its quite a standard skill and in virtually everyones routines. It would be a real shame to lose that option from level 8 routines.
The switch leap is the same, it is a bonus from level 5, and again by about level 7 all the girls are doing it and doing it well. I believe the straddle jump is overrated as a C. In the FIG code of points a straddle jump is only an A skill, we use this code in Australia so it is valued at an A as well.
It is a good idea for getting kids to go to the higher levels so they can do the skills. But it may affect training practises, if kids are waiting this long to do such intermediate skills they will struggle to meet the standards of level 10 and elite in an acceptable period of time. Smart coaches will continue to train the C value skills from around level 7, and at least be introducing drills for them by level 6. But unfortunatly most coaches coach to competitions rather than the gymnasts future. They think about what you need now, for this season and not enough attention is paid to setting them up for long term success.