That is how the system works, conflicts of interest are presumed on both sides. The prosection wants people to say they're guilty, the defense wants people who say they aren't guilty.

The balance of evidence is against the prosecution now, and they can find their own expert witness if they disagree.

That being said, blatantly lying here would be a deathknell for the firm. Every case where they get involved will mean someone brings it up as a way to dismiss their testimony. It would take a staggering payment for most firms to even debate doing something like this intentionally. If it were my firm and I gave up all my morals, I would charge more than I could sell the firm for. I'm giving up basically all future revenue and tarnishing my own name; it wouldn't be something I casually did.