Assertability without Assertion

I think there are cases where one doesn't assert something, but one wouldn't be in violation of any norms of assertion if one did. Probably you can think of lots of cases like that. For instance, suppose that Helen is having a conversation about sloths and their habits. Suppose that she has whatever arbitarily high epistemic access you like with respect to the fact that urinate only once a week, but chooses not to mention this fact, preferring in this instance to listen to the other people speaking instead. If she had asserted it, this would have amounted to an expression of her knowledge or better (certainty, maybe), and it would have given knowledge to her interlocutors, who would have celebrated this fact as relevant and interesting. But she keeps it to herself instead.

I think this is a pretty mundane kind of case—it happens all the time. (There are other kinds of cases with the relevant feature too—imagine a case where one does the wrong thing by refraining from asserting. One may—indeed, ought—to assert, but doesn't.) But I also think it's a counterexample to Rachel McKinnon's 'Supportive Reasons Norm', which she suggests is the central norm governing assertion.

Here is the Supportive Reasons Norm (given on p. 52 of Rachel's recent assertion book):

One may assert that p only if
(i) One has supportive reasons for p,
(ii) The relevant conventional and pragmatic elements of the context are present, and
(iii) One asserts that p at least in part because the assertion that p satisfies (i) and (ii).

My case of Helen is a counterexample because Helen may assert that sloths urinate only once a week, but fails to satisfy condition (iii), since she doesn't make the relevant assertion at all, let alone for a particular reason. In general, that condition will ensure that the permissibility of the assertion entails that the assertion is made. Since it's not true we're only permitted to assert the things we do assert, I don't think condition (iii) is part of a proper characterization of what one may assert. (It is much easier to think something like it may have a role to play in a characterization of when a given assertion is a proper one. Perhaps that's what Rachel had in mind.)