Rudi: 100ppi is the minimum accepted value, not a target or maximum value. Like any printing company, FAA's printers will print files up to the native resolution of the equipment in use, which for inkjet printers typically works out to be somewhere between 240dpi and 360dpi. While the limit that the human eye can resolve in imagery is theoretically around 500 dots per inch, typically a target of 300 dots per inch is considered about as good as necessary for a high-end gallery-quality fine art print.
FAA will print a file up to a size which works out to 100ppi, provided it's sharp and noise-free. However, a file printed at a size that works out to 300ppi will have up to nine times the amount of detail per square inch. If I have a large panoramic image, you can bet that I'm going to make sure that the customer gets that extra detail if I possibly can, rather than scale it down to the bare minimum. I often describe the extra resolution to my customers as the difference between looking at a TV and looking out of the window.
Heather: Try printing one of your own pieces at 100ppi and 300ppi (any size), and see how much clearer the 300ppi image is. That's the reason.