Hi Myrle,
You are correct. You are facing the rule of Simultaneity The Following is taken from the "Little Encyclopedia of Geometric Dimensioning & Tolerancing" by William Tandler Rule 6 Rules of Simultaneity (Y14.5 4.5.12-12.1 pp 68-70, and 5.3.6 p. 92) Lacking the notation "SEP REQT", all features referenced to the same mobile Datum Reference Frame must meet their requirement simultaneously. In the case of separate groups of features reference to identical mobile Datum Reference Frames defined by the second and lower tiers of separate Composite or Compound Feature Control Frames, the Rule of Simultaneity applies separately within each group unless the applicable tier is accompanied by the notation "SIM Reqt" (Y14.5 4.5.12.1 p. 70)
Moot for stationary Datum Reference frames, the rule applies to all mobile Datum Reference Frames, whether due to degrees of freedom not eliminated, to unstable Datum Features, or to Datum Features referenced at and departing from Virtual MMC or LMC. In every situation involving mobile Datum Reference Frames, the full set of "simultaneous" Considered Features participates in constructing the Datum Reference Frame by ultimately limiting its mobility.

When I try to explain this, I ask people to picture a functional gauge that has expanding mandrels (from MMC to LMC for ID's or visa versa for ODs)that mates with all the holes or features of the part in question. It is easy to see that if Datum B were produced at LMC and the 4 holes in your example were produced at MMC, you definitely would not be able to utilize all the bonus tolerance offered by Datum B for all features. I hope this helps!
Bryan Sorensen
Supplier Quality Engineer
Tosoh Quartz Group
Weiss Scientific Glass Blowing
503-605-5667