Subquery in a JOIN not getting restricted

demo.sql

drop schema if exists jaytest cascade;
create schema jaytest;
set search_path to jaytest;

create table questions (
  id int not null primary key,
  user_id int not null
);
insert into questions
  select generate_series(1,1100), (random()*200000)::int;

create table users (
  id int not null primary key
);
insert into users select generate_series(1, 200000);

vacuum freeze analyze;

explain analyze
select questions.id
from questions
join (
  select u.id
  from users as u
  group by u.id
) as s
on s.id = questions.user_id
where questions.id = 1;