Because it is a Prisoner's Dilemma.

Bob can convince Alice to spend half her tokens on proposition B but then betray her and spend all his tokens on B, resulting in 17 votes for B and only 7 for A.

When voting is anonymous there is no way to detect if voters keep their pledge or not, so this should deter them from voting in any other way than in their own interest.

Is the voting in their legislature anonymous? Why?

Unlike ordinary people, the legislators are working for their voters, who deserve to know if they got what they paid for. If I send Bob to the legislature, understanding that he plans to get rid of all the bloody clowns, I certainly want to know if he supported the Bill that gives every Clown $5000 of the city's tax revenue to buy more ludicrous shoes and lapel flowers. If Bob can promise he's anti-clown, then arrive in the legislature and vote a straight pro-clown slate knowing he'll face no consequences, that's no way to run a representative democracy!

Fair enough, that wouldn't work indeed. I was thinking about the abstract case, not actually applied as in the article. It would work with direct democracy though.

Oh no lawmakers actually cooperating to achieve their goals. What a tragedy!

But does not support "across the aisle" cooperation, which is where the concern is.

It usually does result in tragedy, though. It's just reported as "bad luck."

Collusion is a secret cooperation or deceitful agreement in order to deceive others. The plan you suggest is not deceiving anybody. It represents an political agreement and cooperation.

Alice and Bob are making a good deal. Well working democracy should involve maximum amount of deliberation and deal making. It's a good thing. If you want more of that kind of deal making votes should be visible though (quadratic voting between representatives in parliament might work the way you suggest).

I think what would stop it from being effective is that an opposing team could do the same, so they’d have to guard against that. It’s just not a silver bullet I mean.