cucumber-jvm version 3 substitute design pattern for simple table to Map

I was exploring the new io.cucumber.datatable.DataTable support introduced in cucumber-jvm:3.0.0. I have cucumber-jvm:3.0.2 installed.

My existing cucumber-jvm:2.4.0 project makes use of a simple design pattern where I pass a table into a step whose last parameter has type Map. This behaviour is no longer supported in version 3.

So I created a test case to model an alternative to this design pattern. It works, but the Map that is returned is unmodifiable. I worked around it but two questions:

1. Why is the returned map immutable? Am I overlooking an important design principle?
2. Is there a simpler way to accomplish my basic goal of finding a substitute design pattern?

Here is the code:

Feature: Test simple DataTable converters  

  @simple_no_header_map_string_string
  Scenario: Convert a two column table to a Map
    Given a basic street address
      | streetAddress | 333 W Camden St |
      | cityName      | Baltimore       |
      | stateName     | MD              |
      | postalCode    | 21201           |
    When I specify Country "US"
    Then I can print the complete address 


package some.org.stuff.cucumberjvm302testbed;

import cucumber.api.java.en.Given;
import cucumber.api.java.en.Then;
import cucumber.api.java.en.When;

import io.cucumber.datatable.DataTable;
import io.cucumber.datatable.DataTable.TableConverter;
import io.cucumber.datatable.DataTableTypeRegistry;
import io.cucumber.datatable.DataTableTypeRegistryTableConverter;
import io.cucumber.datatable.TypeReference;

import java.lang.reflect.Type;
import java.util.HashMap;
import java.util.Locale;
import java.util.Map;

public class Cucumber302TestBedSteps {

    private Map<String,String> addressMap;

    private final DataTableTypeRegistry registry = new DataTableTypeRegistry(Locale.ENGLISH);
    private final TableConverter converter = new DataTableTypeRegistryTableConverter(registry);
    private static final Type MAP_OF_STRING_TO_STRING = new TypeReference<Map<String, String>>() {
    }.getType();

    @Given("^a basic street address$")
    public void aBasicStreetAddress(DataTable datatable) {
        //Here is the workaround to make addressMap mutable
        addressMap = new HashMap<String,String>(converter.convert(datatable, MAP_OF_STRING_TO_STRING)); 
    }

    @When("^I specify Country \"(.+)\"$")
    public void iSpecifyCountry(String inputCountry) {
        addressMap.put("country", inputCountry);
    }

    @Then("^I can print the complete address$")
    public void iCanPrintTheCompleteAddress() {
        System.out.println("\n\t" + addressMap.get("streetAddress"));
        System.out.println("\t" + addressMap.get("cityName"));
        System.out.println("\t" + addressMap.get("stateName"));
        System.out.println("\t" + addressMap.get("postalCode"));
        System.out.println("\t" + addressMap.get("country") + "\n");
    }
}

I looked at Cucumber-JVM 3 - Convert DataTable to single object using asMap(). I did implement that solution but I have some junior testers so I was looking for something simpler.