If it's a "standby generator" it may need some drain from the battery for the "power failure" circuit to start up when needed. Even if you're not using it that way, there may still be "keep alive" functions that drain the battery for some reason that Honda thinks are "important."
Old people, and other seriously handicapped who need mobility aids will completely understand the complaint.Scooters, power chairs, and the like all use "exotic" batteries that can be difficult to find and "unexpectedly expensive."
A common and relatively inexpensive work-around is a "trickle charger" like what some use to keep motorcycle batteries alive - if it's a 12V battery. A decent one shouldn't cost more than about $20 at Walmart (or another less embarrassing place), although you could look around at the auto parts places and possibly find something "more versatile" at about the same price.
This only works if there's a single 6V or 12V battery of conventional kinds, and most "mobility machines" run off a couple in series (12V x 2 = 24V) or for big stuff that resembles a golf cart, 6 x 6V in series (36 V) is common, and a few even use 90V.
For mobile devices the higher price is partially explainable by the need for "spill proof" features, and the batteries generallly are "gel type" that literally can't spill acid. Motorcycles generally use "wet" batteries, with spill resistant features. Presumably motorcyclists require only that you won't get a face full of acid while your friends have time to pick up your 2,000 lb machine when it lands on top of you in a ditch.
For small portable generators there might be a valid reason for spill resistance, but for those big enough to really have to have electric start it's a little difficult to envision tipping one over. Theres a broad middle-range though where it's a very good idea, even if you might conceivably be able to start it manually on a good day. (You never really need the generator on a good day.)
The question of WHY you "have to buy it from Honda" needs some explanation, but in some cases of configuration, size, capacity, etc, another brand may not be much cheaper even if you can find one. For others, there are a ridiculous number of configurations generably available and a more generic one might be less difficult to find than you'd guess.
A pair of little ones for a scooter, listed at $137 each at a half dozen different "scooter stores," came at $57 apiece about a month ago at the "we'll build any battery you need if it's not in stock" shop and was "off the shelf" in a standard design that met all the specs and fit perfectly.
The batteries were for the scooter that Lin drove into a 5" deep puddle of water, apparently to "see if it would float." It didn't. The old batteries were shot, and wouldn't charge after the blown controller was replaced. Unfortunately, the new batteries (with new controller) revealed that the motor is also crap, so the scooter's still "in maintenance" for now. The scooter shops sell the motor only as part of the complete transaxle ($380? list), but I may be able to find one as a separate component for about $100(?) if the scooter stays on my list of things to do. The batteries will fit the replacement scooter we got for her, if I can keep them alive until it needs them. (The "blown scooter" sells for about $900 - mail order only, from Walgreens or Sams, although the place where we got it went out of business. Repairs are getting marginally cost-effective but I can screw with it indoors instead of mowing the lawn when it's 106F outside.)