if :symbols are immutable how can you use :symbols with default values and then change?

file.rb

@options = {:user => "john",
           :password => nil,
           :host => nil,
           :warn => 100,
           :crit => 500}

opt = OptionParser.new

opt.on("--user USER", "Connect as user") do |f|
    @options[:user] = f
end

opt.on("--password PASSWORD", "Connection password") do |f|
    @options[:password] = f
end

opt.on("--host HOST", "Host to connect to") do |f|
    if @options[:host]
        @options[:host] << f
    else
        @options[:host] = [f]
    end
end