Question

Good morning, Rabbi.
In your series on innovation and tradition, you spoke about how one of the things that characterizes Modern Orthodoxy is that it also incorporates considerations of value-based change into its halakhic reasoning. That is, there can be a certain ruling that stems from some value (or that contradicted a value to which the halakhic decisors in the time of the Talmud were not attentive), but if they were living in today’s reality, in light of today’s sensitivity to different values (the status of women, for example), they too would rule that way.
But then, a few episodes later, you discuss Rabbi Moshe Feinstein’s ruling regarding a Sabbath timer, where he says that if the Tannaim were here today they would certainly disqualify it, and you said that this was not legitimate from the standpoint of determining Jewish law.
I wanted to know what the difference is between the two cases. Why is it legitimate in the modern consideration, but not in the consideration to prohibit? (I assume this is not exactly what the Rabbi is arguing. I would be happy to understand the differences.)