I can appreciate why you are questioning their logic & I don't know if any answer will suffice. Here's some true story's regarding the car manufacturers. The old style push button seat belts were replaced by the vertical push down latch. Why??? It seems that a basic law of physics was ignored. For every action there is an equal & opposite reaction. You can push the button in or smack the back of the latch & the belt will release. People were coming out of their belts in rollover accidents. The manufacturers did not recall all vehicles with these seat belts only those that had reached a court of law. The same was true with the Pinto fuel spilling in rear end accidents. Ford used a similar set up for other makes of their vehicles but few if any made it into court so none of them were recalled. It becomes a matter of $$$$$ & lawsuits.
A while back we ordered a TT that had a maximum weight of 7,000 lbs. When it arrived we took it home & I noticed that instead of two 3,500 lb axles, which is what I thought was on the model at the dealer, they had installed two 3,000 Lb axles. That's 1,000 lbs under the max weight limit. The dealer called the plant, explained the concern & asked for heavier axles & got a resounding NO. I called later the same morning and within 2 minutes was told that new 3,500 lb axles would be shipped within a week. I stopped at the dealer around noon & they all wanted to know what I had said. The reason I got results is because I the consumer figured out what they were doing & they did not want the liability. It turns out that they had about fifteen 3,000 lb axles left over from another trailer run & they wanted to use them. A few weeks later I was looking on the dealer lot & noticed that some new units like ours had arrived. I looked at the sticker on the front left. It said 3,000 lb axles. I looked at the axles & they were 3,500 Lb.
Back to the axles/bearings. The system they typically use is just a hole drilled into the axle & a grease fitting. That's not expensive. What makes the best sense to me would be to go back to just decent bearings packed with synthetic grease. I've explained why they are a problem. Thinner grease & a grease gun that allows you or the mechanic to pump more grease than you need. Maybe they like to use the idea of easy bearing maintenance for the consumer as a selling point for the salesmen.
Liability for them I believe would be minimal. If you over grease, blow out the seals, loose your brakes & have an accident it's your shops fault or yours. You'd be hard pressed to prove otherwise. I just read yesterday of another owner that looked at his brakes after I believe 2-years & three of the four were blown with grease soaked shoes. In all 35 years of teaching I never had anything even close to that happen to wheel bearing seals. There's just no reason for a seal to fail unless it's age, improper installation, or excessive heat due to a failed bearing. I guess sometimes there is just no logic to some decisions. It is however, an interesting discussion.
TeJay