If you are asking about "print shops" in the sense of printing brochures, and other primary text-based material, then yes, I can understand that they only accept PDF: they want the most precise layout possible for text-based layouts (so-called "camera-ready": they can feed your PDF straight into their workflow, generating offset plates from the pages you provide). They will usually not print just one copy, but go for series (up to 1000s of copies).
And for those, jpeg, png and even tiff just aren't going to work: PDF describes a physical page, with precise placement of all elements (including possible crop marks for cutting to size). The others are not in any way linked to a specific paper layout. In addition, jpeg is a horrible format for text.
The confusion stems from this being a photography site, where "print shop" refers to those making physical prints of images only, in various sizes and often on various substrates (paper, simple or fine-art, but also canvas, aluminium, foam core, dibond, etc...). Typical size of a run: 1 copy... Those kind of shops usually do accept jpeg, tiff and png as input, and usually have a standard policy when the aspect ratio of the file differs from that of the requested format.
Photographers don't deal with the other kind all that often, and the requirements for the "input material" are quite different.