it's not so much the case of polite-impolite as a case of personal distance
strangers and people considerably older than yourself you address by Вы and so verbs conguate in plural
a considerably younger person can be addressed by ты
compare to Sie vs du in German
but foreigners are usually absolved from address by ты in the first case
simple people may consider address by ты impolite however courteous you are in actuality