Sentence Correction

Hi

When I learnt grammar lessons, I was taught that a comma does not go beofore a subordinate conjunction. However, I see people placing comma before a subordinate conjunction.

ex: The MPL disputes "urban displacement" based on market logic, wherein private companies connive with local government authorities to keep raising ticket fares until the poor are pushed out from accessible means to reach workplaces.

Can anyone shed some light on when to use a comma before a subordinate conjunction? Likewise, I had read that you need to put a comma before a coordinating conjunction, but I see instances where people don't use one.
Ex: Mass protests began in late May and continued for much of June. In the previous sentence, there is no comma before and. why is it so?

Regards
chinna

???? anyone to the rescue?