It's probably H&K stating a fact about springs that most people don't know and some manufacturers don't care about. A spring under tension will degrade. If it being held under tension within it's NORMAL operating range, the degradation may be so gradual as to be unimportant. If the spring, when compressed (and kept that way), is compressed to the limits of its operating range, it will degrade more quickly.)
It would appear that the H&K design, when the hammer spring is engaged, pushes the spring near to it's limit.
This is why SOME magazine springs,when the mags are stored fully loaded, can degrade more quickly than expected. It's why Wolff Springs recommends downloading hi-cap mags (and semi-compact/compact mags holding a lot of rounds) a round or two for long-term storage. Other mags, like many 9mm 10-rounders, or 7-4ound .45 mags, are likely to never degrade if left fully loaded for long periods -- THOSE springs aren't all that hard pressed, so to speak.
It's been a trend in recent years for gunmakers to have guns do things they couldn't do before -- they've made the guns smaller, but made them function like their full-size companion models. Smaller springs have to do the same work, with less space and less material more and the springs have become renewable resources.
H&K is just telling their owners to do what's best for them and the gun. It may be that the design of other guns use the springs in a different manner, so that leaving them ready to fire is not stressing the springs.