Why is the formatting in my ftplugin applying to all files in every format?

Question

I'm created a custom file format, .cheat, for my cheat sheets, and in that format only I am highlighting alternating rows to make it easy to see which keystroke combo triggers each command. Why is the formatting in my ftplugin applying to all files in every format?

Key lines from ~/.vim/vimrc:

set nocompatible                                                                                                        
au BufNewFile,BufRead *.cheat set filetype=cheat
filetype off 
" vundle stuff
filetype on
nmap <leader>? :75vsp ~/.vim/my.cheat

(Full vimrc below for reference.)

Full ~/.vim/ftdetect/cheat.vim:

if exists("b:did_load_filetypes") | finish | endif                                                                      
let b:did_ftplugin = 1                                       
au BufNewFile,BufRead *.cheat set filetype=cheat

syn match Oddlines "^.*$" contains=ALL nextgroup=Evenlines skipnl  
syn match Evenlines "^.*$" contains=ALL nextgroup=Oddlines skipnl
hi Oddlines ctermbg=DarkGray ctermfg=White
hi Evenlines ctermbg=LightGray ctermfg=White

Full ~/.vim/vimrc:

set nocompatible
au BufNewFile,BufRead *.cheat set filetype=cheat                                                                                                
filetype off 
set rtp+=~/.vim/bundle/Vundle.vim
call vundle#begin()
  Plugin 'VundleVim/Vundle.vim'
  Plugin 'dracula/vim'
  Plugin 'vim-airline/vim-airline'
  Plugin 'tpope/vim-commentary'
  Plugin 'neilagabriel/vim-geeknote'
  Plugin 'tpope/vim-fugitive'
  Plugin 'moll/vim-node'
  Plugin 'danro/rename.vim'
  Plugin 'vim-syntastic/syntastic'
  set statusline+=%#warningmsg#
  set statusline+=%{SyntasticStatuslineFlag()}
  set statusline+=%*
  let g:syntastic_always_populate_loc_list=1
  let g:syntastic_auto_loc_list=1
  let g:syntastic_check_on_open=1
  let g:syntastic_check_on_wq=0
call vundle#end() 
filetype plugin indent on 
set number
set wrap
set breakindent
set autoindent
set shiftwidth=2
set linebreak
set nolist
set cursorline
set cursorcolumn
set ruler
set laststatus=2
set timeoutlen=1000 ttimeoutlen=0
set splitright
hi CursorLine  cterm=NONE ctermbg=Black ctermfg=None
hi CursorColumn cterm=NONE ctermbg=Black ctermfg=None
set updatetime=200
au CursorHold * silent! update
nmap <CR> o<Esc>
nmap <leader>? :75vsp ~/.vim/my.cheat

Answer 1

Your ftdetect/cheat.vim should only contain the following :

au BufNewFile,BufRead *.cheat set filetype=cheat

While the remaining configurations you should place in a ftplugin/cheat.vim :

if exists("b:did_ftplugin") | finish | endif
let b:did_ftplugin = 1

syn match Oddlines "^.$" contains=ALL nextgroup=Evenlines skipnl
syn match Evenlines "^.$" contains=ALL nextgroup=Oddlines skipnl
hi Oddlines ctermbg=DarkGray ctermfg=White
hi Evenlines ctermbg=LightGray ctermfg=White