In addition to @DavidVogt's good answer, I also see an aspect in the difference between werden and werden können in the example sentence. After all, it's not the auxiliary verb werden of passive voice that is left in singular in the example sentence, but the extra modal verb können.
In the simple case of a passive sentence,
In Hessen trinkt man Apfelsaft und Apfelwein.
becomes
In Hessen werden Apfelsaft und Apfelwein getrunken.
Here, the grammatical subject of the sentence is clearly "Apfelsaft und Apfelwein", and it makes complete sense to put the verb in plural. The subject of a sentence in passive voice is simply what would be the object in active voice.
However, if you add the modal "können" and set that to passive voice, you get a strange new implication:
In Hessen kann man Apfelsaft und Apfelwein trinken.
becomes
In Hessen können Apfelsaft und Apfelwein getrunken werden.
While this is a perfectly correct sentence, what it unintentionally seems to imply is that Apfelsaft und Apfelwein are now able to do something, namely getrunken werden. However, it's not Apfelsaft und Apfelwein that are enabled here, they are still only the ones that are being drunk.
To avoid that, it makes extra sense in a sentence with "ge___t werden können" to separate the subject that can do something from the "passive voice subject". A formal or implied subject clarifies this separation:
Es kann in Hessen Apfelsaft und Apfelwein getrunken werden.
In Hessen kann Apfelsaft und Apfelwein getrunken werden.
Es, and also the implied subject in the second line, doesn't represent Apfelsaft und Apfelwein here. It's also not man. It's not even enabled by kann. It's a formal subject much like in the sentence "Es regnet.", with "Es kann...werden." meaning "There is an opportunity to..." in a completely impersonal sense.
Es kann jetzt gewettet werden.
Jetzt kann gewettet werden. (Bets can be placed now.)