Pluralisation of numbers

A comment from a Germanic speaking lady on the morning Midlands news regarding the market - "Three million peoples". Now "peoples" is something that non-native English speakers often use, however what caught my attention wasn't that mispluralisation but the lack of one elsewhere.

When we discuss large groupings we can say that there were hundreds, thousands, even millions yet as soon as it is quantified what happens? Two million, three thousand, four million. What happened to the plural? I've already demonstrated that an "s" can be added so why isn't it?

By the same logic we should discuss cats and dogs, but two cat and three dog :-)