Well that comes back to who is 'they', you take abortion, there are some, some very vocal, persons who oppose any abortion period, even when it would be clear triage, "this baby is dying and taking its mother with it if we don't act now" situations, on the flip side there are those opposed to banning even those abortions which take place so late in pregnancy and are essentially deliveries that one tug would separate a healthy baby from mother. In between those positions are a lot more people who wouldn't want to be classified as 'they' and wouldn't want those lines of reasoning applied to their more rational arguments. You can even expand that to include banning birth control or allowing infanticide, those are rare positions but not unheard of, but very few pro-choicers or pro-lifers would want any association to either of those perspectives. They're not just carrying the analogy too far, they're running on a complete different set of core assumptions.

Same applies to a lot of these groups. Some would favor only eating food grown within effective walking distance, only 'organic' foods, only those plants native to the area, and only vegan ones at that.

This is an entirely different thing then someone who suggests we dump some encouragement money into trying to get more people to keep gardens and chicken coops or try to make it logistically easier for small farmers even down to gardener scale to distribute to local supermarkets. Making greenhouses of any-size tax-write offs, or simply exempt from sales tax, would be an idea (though probably not a great one, just thinking out loud), allowing schools with the space to have gardens for the kids which could fit as a biology/botany/recess sort of setup to greater expose that as a possible hobby and develop basic skills, even possibly as a sport-equivalent, a more physical team or individual thing then AV club, chess team, etc if obviously not track and field or football, akin to 4H clubs. Dumping some money into ads to reminding suburban and rural people that welfare money is typically allowed to be spent on seeds. Maybe even allowing window boxes, trowels, etc to be given away free of charge with a DVD on gardening, etc. Switching a school lunch in my area to be apple or grape juice boxes instead of orange, with a small cash incentive to schools for local acquisition, making that more local and also hopefully at least slightly encouraging future preference. These are the kind of things I mean when I say 'encouragement', and I think it is what most people, even those more vocal on 'locally grown' find palatable and acceptable. I think the 'they' is simply particular loud and crazy and heard more, same as the TEA party members back in 2010 were fairly normal folk but the guy interviewed for the TV was always the craziest, loudest person there.