I _guess_ they are sealable because they are disposable, and the reason most users prefer disposable tubes is -in addition to cross-contamination concerns- that the easiest way of extracting the band is by piercing the tube with a needle.
I often used unsealed (but capped) centrifugation tubes when working with CsCl gradients for RNA isolation. I never encountered any problems with the tubes. Sealable tubes have to be used when working with hazardous material to prevent contamination.