The only question is whether the airlines/credit card companies allow it. As far as I know, they do. So it certainly isn’t a problem.
And even if they don’t, the only issue is you are violating the agreed to “terms of service”. Legally (both halachicly and secular) it is merely a “breach of contract”. That is a civil matter, not a criminal matter or theft. And the remedy the airline/CC company could seek is based what the ToS allow; which is usually cancellation or revocation.
So, in a nutshell, even in the latter case it merely gives the right to the provider to seek remedies (i.e. not honoring the points.) There is no halachic (or legal) violation by anyone.