Nmap uses the --script option to introduce a boolean expression of script names and categories to run. To provide arguments to these scripts, you use the --script-args option. So what you wanted to run was: nmap --script http-default-accounts --script-args http-default-accounts.category=routers
In most cases, you can leave the script name off of the script argument name, as long as you realize that another script may also be looking for an argument called category. For example: nmap --script http-default-accounts --script-args category=routers