The reason is that they are using tile scraping applications that violate the OSM tile usage policy. Typically people just want to download tiles for their city or a vacation destination or travel route in order to pre-load them on a mobile device for offline use.
This is of course a perfect use case for OSM. The problem is that OSM is a project run by volunteers and with limited server capacity. Our primary product is geographic data, not map tiles. So when these tile scraping programs start hitting high zoom levels, it can cause serious problems for the tile server. The solution is to download OSM data instead of rendered tiles. Then you can render your own tiles or even use offline vector rendering.
One tool to make your own tiles is Maperitive. It is pretty easy to use and by default generates tiles that are fairly similar in style to the default mapnik style you see on osm.org although it is very customizable. It is a .NET application although it claims to work in Linux/OSX with mono. Also, while it is free to download, it is not open source which is kind of rare for the OSM world. These two things may cause some people to have an allergic reaction (I'm looking at you, Serge) but if you're still reading, here is a quick lesson on how to use it to render your own tiles. If you keep reading, you should be able to start rendering your own tiles in about 10 minutes. There is a tutorial video and some detailed documentation on the Maperitive website but I don't see a basic text based tutorial that walks you through the common "render some tiles for this area" use case.
Getting Started
Installation is pretty straightforward. Just head over to http://maperitive.net/ and find the "Download" link. Grab the latest .zip file (Maperitive-1228.zip right now) and download it. Once the download finishes, just unzip the Maperitive folder inside of the zip file somewhere and you're done. Open the Maperitive.exe file to fire it up.
