I don't know if this is in anyway a factor but Gregerson reportedly has a monetary clause in his contract for saves. Maybe they are dealing with that.
Per Cot's
Luke Gregersonrhp 3 years/$18.5M (2015-17)
3 years/$18.5M (2015-17)
signed by Houston as a free agent 12/10/14
15:$6M, 16:$6.25M, 17:$6.25M
annual performance bonuses: $0.25M each for 45, 50,
55, 57 games finished. 2017 salary increases by $0.5M
to $6.75M with 100 GF combined in 2015-16
Doesn't have to get a save, but does have bonuses based on how many games he finishes. Saves might be banned in the CBA, I know they frown upon certain stats. To your point, though, Gregerson does have financial incentive to be the last-inning reliever. Doesn't mean he's contractually-obligated to be (he wasn't "named" closer until, maybe, ST), but he probably wouldn't mind the extra cash.