Why do animal cells "mistake" rubidium ions for potassium ions?
There are multiple biological mechanisms that can be brought to bear for distinguishing between atoms. In addition to binding properties (e.g., ionic charge, electronegativity, bond strength), there is also the size of the atoms and even their vibrational properties.
For one of these mechanisms to actually be used, however, it needs to be evolutionarily selected for in some way. The binding properties are by far the simplest to select for, since they are fairly local and permissive in nature, while the others require much more carefully tuned mechanisms.
Thus, we should expect to see most biological selectivity to be developed in terms of binding properties, as seems to be the case. For same-group covalent-binding such as one encounters with oxygen vs. sulfur, there are major differences in the properties of the bonds, giving ample opportunity for biological selectivity.
For Group 1 alkali metals like sodium, potassium, and rubidium, however, ionic bonding is much more uniform. Thus, we should expect mechanisms that apply to one to apply to all unless strongly selected for distinguishing.  In the case of potassium versus sodium, which are both very important in biochemistry, a recently discovered mechanism in sodium-potassium pumps uses binding properties to grab both, then atomic size (via steric hindrance) to distinguish sodium versus potassium as described nicely in the answers to this question.
Presumably, there simply has not been sufficient evolutionary pressure to establish mechanisms for distinguishing rubidium from potassium in the same way.  While sodium and potassium have approximately equal abundance, rubidium is approximately two orders of magnitude less abundant than either. Cesium, in turn, is nearly two orders of magnitude less than rubidium, and rapidly decaying francium mostly doesn't exist.
Thus, unless there is a particularly problematic effect of occasional incorporation of rubidium or cesium, then, we should expect these elements to be essentially indistinguishable from potassium biologically, but not from sodium, since sodium/potassium distinguishing mechanisms will reject them all along with potassium into the &quot;not the right size to be sodium&quot; bin.