Standard sales and cover-your-ass kinda stuff.
They want to sell you twice the battery you need so that they won't have any chance of being wrong.
The big things to look out for in an LI-ION (and its derivatives) battery are the voltages. If you discharge the battery too much, the voltage drops below a certain level. This is bad because it degrades the internal copper electrodes. Same thing happens if the voltages get too high. This damage is not reversible.
Minimum voltage on a lithium ion battery is about 12.5V. 12.8V to be safe. Max voltage is 14.6V. Any deviation from these voltages starts to cause damage. In the case of starting the bike, voltage will usually drop below 12.5V for a little while and these repeated cyclings are what cause the battery to eventually fail. A larger battery has less voltage drop and will sustain less wear over its lifetime, though. All batteries have a limited life span, so this is just part of the game.
The real issues come into play when the minimum and maximum voltages are violated by a large amount or for long periods. Voltage regulators on older bikes are often "good enough" for lead acid, but not suitable for newer battery technologies. Many vintage systems see 16V+ on a fairly regular basis. Likewise, the charging systems for some vintage bikes don't start making enough power to run the bike until 3000 or even 4000 RPM. This means that when you're idling, you're actually running off the battery. If you idle a CB350 with a four cell lithium ion battery for an hour, that battery will be a door stop. But if you put a modern regulator / rectifier onto that CB350, don't idle for long periods of time, and do a lot of your riding on the highway, that same battery may last you three or four years.
The issue has a lot to do with rider habits and battery manufacturers just can't account for every idiot out there. Be smart about it and you can make your own selections.
