I have seen a lot of sentences like the examples below:

1. Todavía no han llegado, su avión debe haberse retrasado.

They haven't arrived yet, their flight must have had a delay.

2. Cristina ha debido de ser una buena profesora, sus estudiantes le han comprado flores.

Cristina must have been a good teacher, her students bought her some flowers.

I respect that Spanish uses punctuation differently, in some cases, from the way English uses punctuation.  However, the Spanish sentences and the English translations use a comma to separate the two sentences in each example (these examples were taken from a quiz on Kwiziq).  For the Spanish, I've checked RAE and I cannot understand why these two sentences are joined by a comma when it seems they should be separated by a period or a semicolon (or even possibly adding a connector or conjunction to join them).  For the English translation, in American English we would have to somehow separate these two complete sentences with some form of punctuation (period or semicolon).  I have also seen similar constructions in other writing, but not usually in newspapers or academic writing.  If you could provide an explanation, I would appreciate it. Thank you.