Hi,
Xilinx and Altera only produce the FPGA, Spartan, Virtex, Coolrunner, etc by Xilinx and Cyclone, Stratix, MAX, etc by Altera. They do not produce the development boards themselves. The job to design and produce the development boards is contracted out to other companies, Digilent and Avnet for Xilinx, Terasic and HiTech Global for Altera. The boards are marketed by not only the company that produced them, but the FPGA manufacturers themselves. Plus as Sckoarn put it, it advertising, some of the peripheral chip manufactures put their name on the board as well.
Hope this helps answer your question.
P.S. No you cannot program a board with the competitor's software! However, there may be third party software companies which can program both, but I can't think of any off hand.