Why did Newt, Tina and Queenie allow Jacob to be obliviated?

Question

In Fantastic Beasts and Where to Find Them, towards the end of the movie MACUSA see Jacob and tell everyone he needs to be Obliviated too. Then they leave and Jacob, Newt, Tina and Queenie are alone when they come out of the underground.

They were all friends (and evidently didn't want him to be Obliviated, as evidenced by their attempt to hide him a few minutes earlier) so why did they have to Obliviate Jacob if the MACUSA had left?

Why couldn't they just Disapparate with him and keep being friends? Would MACUSA have check if he was really Obliviated or is there another reason why they went through with it?

Answer 1

In an unfinished series answers are just guess work.

From what we know most of MACUSA was at the bottom of those stairs from where he walked in to the rain. Knowing his involvement and them not have left the scene its very likely they would have become suspicious if they did not have Jacob walk out. It's also not 100% clear that he did lose is memories, proven in his chose of baked goods. I think the last shot of him with Queenie proves that there is more to it.

Answer 2

MACUSA closely monitors all magic used.

Though Newt, Tina, and didn’t want Jacob to be Obliviated, it’s unlikely they could have tried to stop it without MACUSA catching them. MACUSA monitors all spells cast in America through a map kept on a wall in the Major Investigation Department. Tina would know that, and Queenie possibly might as well, so they’ll know they can’t try to prevent Jacob from being Obliviated without MACUSA knowing.

“Foremost of these is the Major Investigation Department, MACUSA’s centre of control and Graves’ personal fiefdom. On the wall of the department is a map of North America that twinkles every time a spell is cast, pinpointing exactly where.

‘It will only be seen very briefly in the film,’ admits Mina, who nevertheless conceptualized an entire backstory for how the map is being used to spy upon wizard freedoms.”
_{^{- Inside the Magic: The Making of Fantastic Beasts and Where to Find Them}}

In addition to the monitoring of all spells cast in America, it’s likely that what happened to Jacob would be watched especially closely. Madam Picquery knew that they were having trouble letting Jacob be Obliviated, and was also insistent that he needed to be.

“MADAM PICQUERY
Is that No-Maj still here?
(on seeing JACOB)
Obliviate him. There can be no exceptions.

MADAM PICQUERY reads the anguish in their faces.

MADAM PICQUERY
I’m sorry – but even one witness . . . you know the law.

A pause. She is uncomfortable at their distress.

MADAM PICQUERY
I’ll let you say goodbye.

She leaves.”
_{^{- Fantastic Beasts and Where to Find Them (The Original Screenplay)}}

It’s likely MACUSA would have been watching to make sure no one tried to escape with Jacob. Aurors were right outside the subway where Jacob was supposed to exit into the rain.

“JACOB leads the others up the steps of the subway, QUEENIE following close behind him.

Rain is still falling heavily, the streets now almost empty but for a few hard-working Aurors.

JACOB has reached the top of the steps and stands, gazing into the rain.”
_{^{- Fantastic Beasts and Where to Find Them (The Original Screenplay)}}

Newt, Tina, and Queenie likely realized there was nothing they could do without MACUSA finding out, punishing them, and still Obliviating Jacob despite their best efforts to prevent that from happening.