(Funny thing is, in the books they weren't actually chucked straight in the prisons, they were only locked up after they started being rude and disrespectful to the Elves...also the conversation between Thranduil and Thorin goes slightly differently, so I'm going to do it sort of halfway between here if that's alright because the book version actually makes more sense xDD)

Maleficent wasn't allowed to stay with Kili for long as Mel's hand rested firmly on her shoulder and forcefully, however still rather gently compared to how rough the other Elves had been, steered her in the same direction Thorin had been taken. The Elvenking's voice echoed towards them as they climbed the staircase towards his throne, where Thorin was standing before him. Two guards stood off to the side, spears at their sides, and silent as they watched and waited in case the dwarf needed to be removed.

"Some may imagine that a noble quest is at hand," Thranduil began as he paced from his throne towards Thorin, who glared at him hatefully, "a quest to reclaim a homeland, and slay a dragon. But I myself am not so sure. Now, you will tell me exactly what you are doing in my woods, and to what purpose, or you will end up just like your friends. You plan to steal something that does not belong to you, I would deem." he nodded curtly towards Mel, who gave her uncle a nod in return, as the she-elf arrived with Maleficent.

"We were just passing through." insisted Thorin stubbornly as he avoided the elf's piercing blue-grey eyes. "We were hungry, and looking for food and shelter." he certainly wasn't going to admit to the Elvenking that they were going to the Lonely Mountain, or he would most certainly demand some sort of bargain from them. 
"I do not believe you." Thranduil answered sharply. "Tell the truth, or I [i]will[/i] lock you up until the fancy strikes you to stop lying to me, even if it takes a hundred years!"