Though BrowserUk makes an interesting and rigidly logical argument, in reality most people don't think the definitions through, they just follow common usage as they interpret it. So, in spoken language, common usage trumps all. It redefines academic convention and historical definitions. This is just how languages naturally evolve.

That said, behavior is synonymous to expected behavior in common usage (at least in U.S. English), thus its application in present and future tense is (IMHO) perfectly acceptable.