In coin flipping protocols, why aborting is allowed, and why the non-aborting party flips a coin at the end? To convey how an adversary can bias the coin, most often a simple commitment-based two party coin-tossing protocol is given, as in [1]:


Alice sends Bob the commitment $c = commit(x)$
Bob sends Alice the bit $y$.
Alice sends Bob $decommit(x)$, Bob verifies Alice actually chose $x$.
Both compute result $r = x \oplus y$ 


It is said that Alice can place a bias of $1/4$ towards her choice after she receives Bob's bit $y$; if the result is what she wants, she plays along, if not, she aborts, forcing Bob to flip a coin on his own, which has $1/2$ chance of her choice coming up.

I understand the protocol and how a bias can be exerted. What eludes me is why such protocols are of interest:


Why is either party allowed to abort at will? The only point abortion makes sense is when Bob fails to verify, but they can abort whenever they want. If only such an abortion is allowed, the bias-by-abort scheme fails since Alice cannot abort upon seeing the result after the second step.
After Alice aborts, why does Bob flips a coin on his own? If the coin was flipped in case of a dispute, Bob can choose the way it suits him. If Bob could've just generated a random bit without having to prove anyone it is not biased, he would not have bothered with the protocol in the first place.


[1]: https://eprint.iacr.org/2012/643.pdf - Section 1, Example 1.1
1.
You usually allow parties to abort at will at any stage of the protocol since a malicious adversary can always do that in a real protocol execution, and therefore this is not something we can enforce. 
A bit more formally, the best we can simulate (for a tutorial on simulation-based proofs see this) is an adversary that can abort, since that's unavoidable anyway.

2.
I didn't read the details of the paper but I assume that in the description, Bob still throws a coin if Alice aborts. 
This is done precisely to break the bias imposed by Alice with the abort.
This works because Bob is honest (since Alice is malicious as she aborted) so the output, that is, the coin tossed by Bob, will have a uniform distribution.