Unethical ? 4 years for EOS is pretty standard in the consumer industry. It's not like it will stop working anyway, it will go on working fine with Lion.
No but Apple stops updating it and pushing out security updates for the older OS so it could also be said by not updating your computer becomes less and less secure.
Like I have said before it is not the fact that Apple is not supporting older hardware on the new OS but the fact that Apple pretty much drops near all major support as soon as the newer OS comes out. This include security updates and the fact that software is pushed to really only run on the newer OS.
Take for example iTunes. It works great on an OS from 2001 for windows but it requires an OS from 2007 for OSX. This lack of support also means that people can not use their iPod or other Apple products with out updating. How long until iTunes will require 10.6 and be Intel only (my guess is not long after ML.