You'd think that employers would be all over UBI then because it means that they can instantly cut the wages of their workers by the amount of the UBI and that more people will have money to purchase their goods and services.

Companies that depend on underpaid workers will fight this tooth and nail.

Why? It means that they can pay less than they do now and still retain workers. How is it not a positive benefit for them? It potentially means that people working for them actually want to work and aren’t doing it just to survive.

Because the jobs they're offering are shitty, demeaning and menial, and most people who take them are doing so because they need to survive. Absent the survival need, the companies will have to dramatically improve working conditions to maintain the sheer quantity of labor they require.

Perhaps, but the reduced wages (since total income is now UBI + wage instead of 0 + wages) sounds like it will more than make up for whatever increased costs they may have to make the working environment less shitty.