The 4th amendment protects against unreasonable search and seizure, and the state must show cause for conducting any search. At a border crossing, search for contraband (including contraband plants and animals which can seriously damage the United States) are an exception because it's reasonable to search people at a border crossing.

https://en.wikipedia.org/wiki/Border_search_exception

Here, the EFF has to show that the search that was conducted wasn't reasonable in some manner or that it otherwise violates due process. They will need to show that the execution of the search wasn't reasonable, because being subject to a search in and of itself generally is. The only SCotUS ruling on search of electronic devices has been Riley v California (https://en.wikipedia.org/wiki/Riley_v._California) where they ruled that a search warrant was required to search an electronic device even when an individual has already been placed under arrest.