If the religion itself doesn't support the decision, how do they justify the proposal?
I think the history of religion is replete with examples of people "finding" things (always consonant, somehow, with their preferences) not explicitly forbidden to be mandatory, and also "finding" things (always consonant, somehow, with their sense of disgust) not explicitly approved to be forbidden. When the vast majority of the members of a religion do not know the texts upon which their religion is based (either because of illiteracy, or simple time pressures, or because they are told by the powers-that-be that they shouldn't; always my favorite), you could tell them pretty much anything and get away with it.
There are a lot of women in Saudi Arabia (+11.8 million according to Wikipedia). This proposal was from a spokesperson for Saudi Arabia's Committee for the Promotion of Virtue and the Prevention of Vice. If it were to pass, would it not apply to all women in Saudi Arabia?
Quite so, and that's ghastly. However, there are 110 million Muslim women in Indonesia who wouldn't think twice about slapping a guy for even suggesting it. Hence, it is a "Saudi cultural" problem, not an "Islam" problem.
To be clear, my objection is not that such a notion isn't immoral or retrograde or misogynistic, nor is it that Islam doesn't, quite on its own time, present all of those unseemly habits (unethical, backward-looking, and woman-hating) in other ways. It's only that this particular example isn't a very good one if the intended target is Islam and not the peculiar idiosyncratic backwardness of one (comparatively) small part of the Islamic world.
[Tried to post this last night; server wouldn't let me. :( ]