Many abstainers probably prefer the idea of consolidation, but there are concerns about union fees, job changes, etc. So, not completely opposed. On the other hand, fear votes yes, as Amazon may retaliate and close the facility.
Also, more likely to believe their vote was not confidential, and that Amazon will somehow know how they voted.
Abstaining from allows a person to tell co-workers, friends, family, etc. (or simply rationalize themselves) that they have supported the consolidation effort while informally doing so to avoid upsetting Amazon.
An unfavorable situation for the worker. Trying to consolidate multiple Amazon facilities at once would be more efficient, but seems impractical in practice for legal and logistical reasons.