Practical vs ideal lowpass interpolator
When designing a filter, you really care about its behavior in two regions:


Passband: You want little attenuation in this region, and maybe other properties as well, like linear phase, depending upon your application.
Stopband: You want as much attenuation as needed in this region.


Between these two is the transition region. This is treated as somewhat of a "don't-care" band. You don't typically constrain the response too tightly in this area, so you can't really count it being usable. In your example, the passband lies below $0.45 f_s$; after the interpolator, you only plan on using frequency content below this threshold.

This means that you can allow some aliasing in order to simplify your filter design. Your transition region starts at $0.45 f_s$; everything above that frequency in your filter's output will either have a response that is unpredictable given your filter specs (if it lies in the transition band) or one that is highly attenuated (if it lies in the stopband). The takeaway: 


  You can't rely upon any frequency content past the passband edge in your filter output anyway. So if it makes the filter design cheaper, why not allow the frequencies above the passband edge in the filter output to contain aliased garbage? 


This technique is commonly used in multirate filters as you've noticed, as it allows savings in the required filter order in order to meet a given set of passband/stopband specifications.