Mumofstig
I think it's very much a case of erring on the side of caution, that and liability. I know it's done differently in different parts of the world and this is just how it's done here.
Tested recipes are ones that have been formulated tested and studied by an extension office or other governmental body that deals with food safety. They determine ingredient ratios, which I believe are dictated by acidity of different things used, and they've also tested processing times to ensure that any undesirable bacteria or nasties are killed off and that the final product maintains (as much as possible) the desired texture and nutritional value.
Using recipes or even techniques that have been passed down from generation to generation are not usually recommended. For example with tomatoes, lots of folks say that their parents or grandparents never added acid to tomatoes. It's argued however, that the tomatoes we grow now a days aren't as acidic as the ones that were grown all those years ago, consequently the emphasis on the addition of acid.
I do think it has a lot to do with liability. The company that manufactures the canning supplies also puts out a book on preserving. But we also have the national center for home food preservation and innumerable university extension offices that offer tested recipes as well.
We do process everything in a boiling water bath or a pressure canner. Vinegar, sugar and salt act as preservatives but processing helps sanitize everything so no "baddies" get preserved. There are quite a few that don't follow the recommended guidelines since that's the way they've always done things and no one's ever died. I didn't grow up canning, so I can't find that argument useful. I prefer to err on the side of caution and use the scientifically tested and approved recipes over anecdotal evidence.
Like I said before though, that's just how things are recommended they be done here. That said there there a lots of people who deviate and use other recipes and techniques that they are more comfortable with or more experienced with.
This doesn't by any means invalidate how things are done in other parts of the world and I would love to hear Tosca's take on canning/bottling and the techniques and recipes she uses. =)
Comfreypatch
Bottled lemon juice has a consistent ph and so that is why it is called for instead of fresh. When a recipe calls for vinegar, sometimes it will even list the percentage of acidity it should have.
It can seem overwhelming and some of the steps can seem superfluous, but it's worth it when you consider all the time, effort and energy that goes in to it.
Happy Canning