If the free parts for life is a scam marketing tool for some, it isn't for others. I will take my regs in for 2 year service intervals.
Typically the free parts programs have required annual service, but lately a few of the manufacturers are switching to every two years. It's still a scam, and here's why:
First, for 'average' recreational use, meaning 50 dives/year or less, regulators that are well cared for can easily go far longer than 2 years between service; maybe 5 years or more. About the only thing likely to need some attention might be the 2nd stage seat, and that's likely just an adjustment. Of course, not everyone cares for their regulator well.
Second, the true cost for these 'parts' is mere pennies. Maybe a dozen o-rings that are certainly less than a nickel each at wholesale, and a couple of plastic seats that are likewise very inexpensive to produce and inventory.
A big part of the scam is the gall that regulator companies have in charging $20-30 for the rebuild kits which consist of these parts. This artificially inflates the value of the 'free' parts. Once you find low-cost sources for the o-rings and seats, parts cost for rebuilding many regulators is only a couple of dollars. Some 2nd stages are far less.
What would be 'reasonable' or 'fair' pricing for regulator service would be about $5 for a parts kit, maybe $25-35 in labor, per stage, as needed. For most recreational divers this would be once every several years.