Why does Ruby modify frozen array?

frozen.rb

a = [
  { entity: [1, 2], other: 1 },
  { entity: [2, 3], other: 3 }
]

def copy_examples_to_entities(array_of_examples)
  array_of_examples.map do |example|
    entities = example[:entity]

    if entities.count > 1
      entities.map do |entity|
        example.tap { |ex| ex[:entity] = entity }
      end
    else
      example
    end
  end.flatten
end

a.freeze
puts "Frozen? #{a.frozen?}"

copy_examples_to_entities(a)
puts "Frozen array == original array? #{a == [{ entity: [1, 2], other: 1 }, { entity: [2, 3], other: 3 }]}"

array = ['ahoj', 'baf']
array.freeze
# array << 'neco' # => RuntimeError: can't modify frozen Array
array.first.concat ' Tomasi'
# array # => ["ahoj Tomasi", "baf"]
arrray.first.freeze
array.first.concat(', freeznul jsi sice array, ale ne objekty v ni, ktere pokud nejsou frozen, tak jdou stale modifikovat') # => RuntimeError: can't modify frozen String