Your point is not totally without merit, but...
Morrison is actually guaranteed 6.5 million.
I am guessing that Morrison's value was hurt by concerns that he was a one-year wonder, as his career numbers would suggest that possibility.
A highly credentialed all-hit no-field guy can still make decent coin, as in J D Martinez.
The bottom line with Hanley is, he still seems capable of putting up a 3 WAR season.
And when you're the filthy rich Boston Red Sox in pursuit of a championship with a small window, you can't be concerned with f***ing around with whether Hanley's option vests. You go all out to win this year because that's what your fans expect and deserve, and if the option vests, that's show biz.
I'm fairly sure a Yankee team in a similar position would do the same.