Greg Johns of seattlepi.com has the details. Basically, the Seahawks heard enough feedback from the NFL regarding Hill's off-field troubles to feel comfortable welcoming him back at this time.
It still doesn't add up entirely from my perspective. Nothing was preventing Hill from practicing in recent months. Imagine how much further ahead he might be if he had participated in practices to this point. If having players practice for the last couple months wasn't a big deal, then why have practices at all? The team obviously considers them very important.
Anyone familiar with the NFL's approach toward discipline could feel confident in projecting general parameters for punishment. A suspension to start the season seems likely. It' not like the league was going to banish Hill for a full season or punish Seattle for letting him practice.
The Seahawks can come out ahead anyway if their banishment of Hill motivates the linebacker to stay out of trouble and play at a higher level this season.