Scubapro does restrict parts sales in the US - but on the other hand, it is rare to not find a parts kit you need on e-bay and if you can't find one, you just need to wait a week. Scubapro has a large dealer network and dealers are always coming and going and the ones that go will sell things on-line. Scubapro as a company has a long history of supporting discontinued regs and it is common to see 40 year old Mk 5 Adjustables still in use.
That is an important point as while another company may sell regs and sell parts kits direct to the consumer, the odds are good that they are selling a liscensed copy of another reg design and selling them in comparatively low numbers, so once that design is discontinued, it becomes very cost prohibiltive for a small company to provide long term parts support. So even if they have a policy of selling parts directly to divers, and even if they have the willingness to keep doing it, they may not always have the ability to keep doing it. Another thing to consider is that your current LDS may drop the brand you currently own and once again, you may lose access to parts unless the company will also sell parts direct to the diver on-line.
Between Marci and I, we have 4 backgas regs, 2 deco regs and 4 stage regs that see regular use, and there are a few more that are either spares or see intermittent use. That's a lot of regs to maintain and very cost prohibitive if you have send them to a shop.
That is exactly why I prefer Scubapro. Annual service parts are readily available on e-bay and if something major happens on a trip, you can still get them serviced almost anywhere. I also have a very high degree of confidence that Scubapro will still be making those parts 10 years from now for my "older" Mk 10 G250 deco and stage regs, and will be making parts 20 years from now on my Mk 17 G250V back gas regs.
Aqualung also has a history of long term regulator parts and service support, but they also restrict parts sales and it is very rare to see their parts available on e-bay.