It's just cheaper to use a single shield bearing, as already noted. You can't easily buy a single shield bearing, the shop will tell you to just pry one off it you need to; you can order one but the special order price will kill you on buying one or two. Honda, buying thousands at a time, can get whatever they want and the single shield will cost less than double.
Honda also uses lip seals on the wheels - one in the retainer cap and another on the sprocket side or speedo drive - so they don't use sealed bearings - again, the cost saving is worthwhile and the extra friction drag is undesirable.
I always use top brand double sealed bearings on wheels. The cost increase is negiligible and the bearing will retain its grease basically forever. The extra friction is hardly noticeable, and you can lose the spacer lip seals if you like (keep the seal on the speedo drive though).
I've found the most important thing with these wheel bearings is the fit. Honda was pretty sloppy with the hub boring - some wheels, probably done with new cutting tools, are a perfect fit on the bearings. Most are very tight, I think because the cutting tools wore down a bit and left a smaller hole.
The hubs all have one retained bearing and one floating bearing (the extra bearing on the bigger bike's sprocket carrier not included as it doesn't carry the bike, just the chain force). The floating bearing MUST be a sliding fit - pretty snug but if you need to press it in it is TOO TIGHT. At least half the wheels I've done need some work on the floating side's hub bore with sandpaper to get a proper fit, I have never found one too loose. If it is too tight you will have premature bearing wear because there will be a high axial load on both bearings. The floating bearing has to be able to shift side to side a bit to relieve that residual axial load. the retained bearing can be pressed in if need be, it just has to be fully seated with the retainer cap snug and staked (or loctited).
Until I figured this out I was occasionally replacing bearings a few months after doing them because the wheel got very stiff to turn. The bearings were either cooked dry or even had shattered balls and cages. I knew I hadn't abused them on assembly. I realized the failing ones were really tight fits, studying the hub and some bearing manuals led me to the solution. Single row ball bearings do not like much axial load at all, speed and load ratings drop precipitously with any axial load.
