The key distinction is that from the NLRB's perspective, lockouts are legitimate. They're identical to strikes, except they're perpetrated by management and not labor. But there's nothing a priori unacceptable about a lockout, since one can always be avoided (and ended) by negotiating and it's the NLRB's goal to encourage such. The NLRB, however, has a vested interest in preventing sham-decertification-coupled-with-lawsuit as a negotiating practice since it is an inherently unfair negotiating strategy (and it ties up federal courts with an issue that should really just be resolved in a board room).
In principle, if a union should be allowed to strike then management should be allowed to lock out labor. If one isn't allowable, then the other shouldn't be either.