For starters, it isn't up to the State of Washington to "keep them here" or not. It's up to the feds.
Second, all this is doing is making sure the feds don't misinterpret what the crime was.
The Feds consider any crime punished by a year or more to be an "aggravated felony."
At the same time, most lower courts in Washington give all mindemeanants the maximum sentence basically every time (90 days for misdemeanors and a full 365 for gross misdemeanors). They usually suspend the bulk of it (or all of it) but they hold the remainder over the offender to force compliance with whatever conditions they impose.
This creates an unjust situation whereby the feds consider most any gross misdemeanor to be an aggravated felony, even where the crime isn't a felony at all. It makes no sense. How can a gross misdemeanor be an aggravated felony? For immigration purposes, a felony where someone does 9 months in jail (say Assault in the Second Degree) is better than a gross misdemeanor where he does no jail but has the 365 suspended (say, littering more than a cubic yard). That's what's wrong with the way it was.
It isn't that we necessarily want to keep or deport anyone. We just want the law to be fair, and clear, and logical.
It's stupid. It makes no sense. And it ties the hands of the immigration courts. It isn't that they can't still deport criminals, it's that the way it was before, they had to deport some criminals, while having the option to deport or not deport other, worse, criminals.
All this law does is ensure the Feds don't wrongly misinterpret what happened. They can still deport any non-citizen for any number or reasons, but their hands are no longer tied because of this weird quirk in the law.