Apple could in theory offer a "bad reception" switch for people who are exceptionally risk-averse and/or distrusting of scientific evidence (even if that evidence is accepted by literally every country on earth, which Roundup is not).
But that would have absolutely nothing to do with people's level of sensitivity and resilience. People have varying levels of sensitivity to peanut compounds, volatile organics in perfumes, and H2 blockers. People have varying levels of resilience from a flu infection, muscle strain, and the side effects of alcohol. All of those things are easily measurable, both subjectively and objectively.
Tissue response to low levels of non-ionizing electromagnetic radiation is not going to vary substantially across the human population, and even if it did the "resilience" to it would be utterly impossible to measure in any meaningful way outside of exposure to blatantly hazardous levels.
The point, however, is that regardless of what nation you live in--from the most strictly regulated to the most lax--none of them have a problem with the level of radiation cell phones put out, and those devices are carried daily by roughly three quarters of the planet's population. So there's essentially no point in offering a "make phone worse" switch to appease a tiny, unusually paranoid fraction of the population, when those people wouldn't actually be satisfied by that solution anyway.