Tom:
(Note what I say here has to do with conditions where I live and it may be different in other places but I think the business logic still applies)
Law firms set their own rates. If you are doing IP law, you'll find that many of the firms look for people with related degrees to the subject matter because they can charge more (because the greater understanding of the subject matter can impact success in litigation, in patent filing, in patent defense, in overturning patents, etc). I know another lawyer who has a biology degree and works in biotech IP.
It is not a *necessity* to have the actual technical or science degree, but it does provide a depth that those without the degree (even if they took a law degree with a focus on IP law) won't have.
They get more money because their law firms recognize the value of having that extra depth in the subject matter. It's not that they get more money as a demand, but their increased competence in the core technical or science area they are working with makes them also better capable to achieve success with legal proceedings. And the law firms recognize that.
Most of them didn't start out to be lawyers - they went into science or a technical/engineering field. They just gravitated later into a law career and were smart enough to pick a branch of law which lets them leverage their prior science or tech experience. And the market has seen to it that they make more money because of that complimentary degree.
I am fairly certain specialized lawyers of any sort cost +50-150% what a general lawyer would cost (my experience in some other domains of law plus talking to my friends who are lawyers with tech or science backgrounds). And I'm fairly certain my friends' existing complimentary degree probably got them 20-30% more money than they would have gotten in salary if they had a non-complimentary degree (or they might not have been hired by firms who want the best people for the job which let's them charge closer to the +150% price point for their firm's services).
There is no magic "I have X, I get +Y%" in the real world (some union contracts or gov't contracts excluded as some have specific language around some things that will give fixed increases). In business, it's "How much can a service company charge?" and "How does having a particular skill set from an employee increase that?". That plus some haggling and a widely qualified individual can drive a good bargain.
For instance, my first 5 years working, I had over 8% a year pay raises. I think the highest was +16% one year. Part of that was the insanity of tech wealth at the time. But part of it was also I went entirely prepared for salary negotiations each year (with data from many others in my industry in the area that I knew). I know other software developers who did not or could not come armed with that, and they got 3-5% raises. Some were shocked at how much more I got than them, but I was only ensuring I got what others I knew of in the same sorts of work at other companies got.
What is possible for a salary is a product of the market. Your actual salary though is largely determined by your desirability as an employee and your ability to not be low-balled by management that tries to do that all the time just to save money (and many developers never question the raise amounts). And when I got team leader training, that helped me get a larger raise as well. The more pertinent skills you have, the more you are worth.
The why for that part is simple: If I want someone with a broad range of skills (often true in tech companies that help other companies because they need skills all over the map for that business line), and someone else has trained them, I can pay them more than their employer and usually they'll jump ship for more money. So there is, in good business times, a competition for employees with key skills and the amount employers are willing to pay is driven by the bargaining and the width and breadth of a skillset shows up there.
TomB