The actual goal of fasting is to stop us from sins. If a person does not refrain from sins even while fasting, then though the condition of compulsory worship would be fulfilled, he would not be entitled to the rewards (Ajr) of fasting.

Hadith:

Prophet of Allah صَلَّى اللّٰهُ عَلَيْهِ وَسَلَّم said:

If a fasting person does not stop lying or cheating, then Allah is not in need of him to refrain from his food and drink.

Sahih Bukhari :1903

Hadith:

Prophet of Allah صَلَّى اللّٰهُ عَلَيْهِ وَسَلَّم said:

There are many people who fast but gain nothing except for hunger. There are many people who pray at night but gain nothing except for a sleepless night.

Ibn Majah:1690

This shows how important it is to refrain from sins.

Important Points:

• We are in the state of worship while praying; from Takbeer (اَللّهُ اَكْبَر ) to salam. Similarly, we are in the state of worship while fasting; from Fajr Adhan to Maghrib Adhan (sunset).
• The duration of fasting is the longest amongst worship. So, in the state of worship committing sins, listening to songs, back biting others or doing other immoral things is Haram (forbidden).

Fasting does not mean staying hungry or thirsty only; It actually means to refrain from sins. 

واللہ اعلم و بالصواب