This is comparing actual pleasure and pain with expected pleasure and pain. It says nothing about why actual pleasure should be capped but actual pain should not.
Even those sentences don't argue for a general cap on pleasure but not on pain. In fact, it's hard to come up with any general claim that they do amount to an argument for. The specific example given, one animal eating the other, is obviously asymmetrical, so of course we would expect the respective valences of pleasure and pain to be asymmetrical too. But that's a particular property of that specific example.
Well, I think you'd have to accept the Schopenhauerian worldview (or something similarly pessimistic) in toto, that sentient life involves a surfeit of suffering and conflicting desires that cannot possibly be reconciled, in order to accept the general asymmetry. A short quotation can, of course, only give a glimpse into that: it should be understood as rhetoric more than an analytical chain of reasoning. One animal eating another is absolutely central to the system of nature, since it's how the system sustains itself; the pain of one creature being hunted down and eaten is necessary for the other creature to survive, and yet the pain of dying is obviously greater than the satisfaction of eating.
