my thought on this is that there is massive value in simply being able to take advantage of dual-wielding's faster recovery (in addition to the two weapon style extra recovery). shield style is not really supposed to a DPS style, and making shield bashing more weapon-like makes it do too much IMO. I definitely use the bashing shields in all my runs (especially the medium shield fire one, it's so easy to get), and it's frankly very rare that I use anything else.
i don't know if it's an oversight, but I prefer the status quo and think the monk shield is overtuned from benefiting from transcendent suffering - it would be a good-enough shield on its own if all it did was shield bashing and benefiting from transcendent suffering, because even with less base damage than monk fists, getting +16 deflection w/ weapon and shield style (up to +20 at mythic) and +1 engagement while still having dual-wielding speed is an interesting and really good trade-off. But Tuolito's Palm does a whole bunch of other stuff as well. (It would still be an interesting and potentially good trade-off if it merely did all the other stuff and didn't benefit from transcendent suffering.)