発売する is “to offer for sale,” so 発売される is “to be offered for sale.” Whether it appears in the active or the passive depends on the subject of the sentence, which is marked by が.
If the subject is the person or entity offering something for sale, it will be used with する. If the subject is the item being offered for sale, it will appear in the passive, with される. (In English, it’s the same-- “to offer” is active and “to be offered” is passive.)
Here, the subject of the verb is the item being offered for sale (「…チーズタルト」). So it appears in the passive.
Consider this example of an active sentence, courtesy of Jisho:
My uncle’s company launched a new product last month.
In this case, the entity doing the offering (会社) is the subject, so the sentence is active rather than passive.
Edit: I just saw your follow-up question regarding the verb at the end of the sentence. If you look at the translation of the sentence, the intended meaning here is “we have decided” to sell the cheese tarts. Again, this is an active sense, because the subject (we) is performing the action (deciding).
In Japanese, personal pronouns are often if not usually ellipsed, so there isn’t an explicit 私達が to act as subject of 決定. It’s up to the listener to interpret from context what is meant. But the use of する rather than される gives an important clue to the meaning of the sentence— する indicates that the speaker has made a decision, or is reporting that someone else made a decision, to sell the tarts. される would have the sense of “it has been decided” that the tarts will be sold— leaving it vague as to who decided it, and potentially indicating powerlessness over, or discontent with, the decision.