"Why are weapons prohibited?"
Translation:Waarom zijn wapens verboden?
Because they are dangerous to hold and because Dutch (and other countries in general) don't feel like it is necessary to have a gun in order to protect yourself.
I hope that you speak English well enough as to be able to understand what I'll say next, since it's a bit to complex for me to try to explain it in Dutch (I could try if needed, just let me know).
I think that the confusion here arises because you're mixing the passive voice with the copulative use of the verb zijn/to be.
While in the passive voice we use zijn as an auxiliary, in this case what we have is the verb zijn as a main verb plus a past participle verb acting as an adjective.
At first sight, such a sentence (both in English as in Dutch) may be confused with a passive voice sentence:
Waarom zijn wapens verboden?
Why are weapons prohibited?
As a statement:
Wapens zijn verboden omdat ze zijn gevaarlijk.
Weapons are prohibited because they are dangerous.
Here the verb zijn/to be is the main verb of the sentence, and the past participle is acting as a predicative adjective. I think a good test for this is replacing the past participle verb with an adjective and seeing if in that context the sentence still makes sense.
Waarom zijn wapens duur?
Why are weapons expensive?
Wapens zijn duur.
Weapons are expensive.
In such a case, there's no way we can add an agent (a 'doer'/'senser'/'sayer'/'experiencer', etc.), because nothing is being done (sensed, said, exprienced, etc).
Now, the sentence you proposed was:
Waarom worden wapens verboden?
In English this also translates to
Why are guns prohibited?
In this case we are in the presence of a passive voice sentence. Here, the verb worden/to be is an auxiliary verb and the past participle is the main verb of the sentence, which in combination with the auxiliary form a passive sentence. A way to test this is seeing if there is an agent (introduced by door/by): if there is an agent, we can be certain that it's a passive voice sentence. Sadly, in this sentence there's no agent, so we cannot be sure what the 'sayer' of this sentence had in mind.
We can try replacing the past participle and checking if the sentence still makes sense. But sadly we don't have enough context as to be 100% certain. Thus, we'll have to conclude that, as we're not studying the passive voice in this skill, it's not possible to use the passive voice.
Remember that in Dutch we use worden and zijn (and their past tense counterparts) as passive voice auxiliaries. Roughly, they are to be translated into the following tenses of the verb to be :
Word(en)= simple present/present continous (am, is, are)
Werd(en)= simple past/past continuous (was,were)
Ben/bent/is/zijn= present perfect (have/has been)
Was/waren= past perfect (had been)
Hope this helps!
PS: due to all the arguments I presented I think you should report this sentence, since there isn't enough context as to be sure whether Why are weapons prohibited? should be translated as a passive voice sentence or not.