The stance that they don't have 'inner lives similar to humans' could be seen as going against Occam's Razor—for some definitions of 'similar' of course (we wouldn't expect them to have linguistic thoughts for instance, but it might be fair to say their experience of temperatures is similar). This is the same basic inference we use to determine that other humans also have inner lives: we observe correlations between our inner state and external states of ourselves and others—others appear to be the same sort of thing on the outside, and we understand how external changes are dictated by internal changes in ourselves (e.g. facial expressions changing with emotions). It would be adding unnecessary complexity to say that what causes all of the similar external changes in others doesn't have the same internal 'stuff' driving it as in ourselves. What justification is there to think I would be set up in one way completely differently from everyone else? For (many) animals, you also have external behaviors that map roughly to external human behaviors
driven by certain internal emotional conditions (e.g. when presented with a threat, or when feeling relaxed, etc.). I think it's a better hypothesis (not proof of course), that animals also have similar inner lives (again, for an appropriate definition of similar...).
Probably the bigger thing here, though, is that questions about 'inner lives'—if we're talking about subjective experience—are outside the purview of biology anyway.