I've seen this at a few fairs and markets this summer; I'm told it's quite common. I don't can (or jar) because I don't get produce cheap enough to make it cost-effective, but everything I know about canning tells me that this is not food safe. Those jars of lemon curd (in this case; I've also seen it done with marmalade) are covered with cling film (saran wrap) and held on with rubber bands. I'm glad they had those out there - else I may have bought something from them. If they don't realize how unsafe that is, who knows how the jams have been treated!
(If you don't know what lemon curd is,
here is my favorite recipe to give you an idea. It's pretty much the lemon part of lemon meringue pie).
Bakewell Show, 4 Aug 2011
