Also unlike the overpass (unless it is tolled) the resources extracted and moved along the pipeline continue to generate revenues for the federal and provincial governments.  

In the case of the TransMountain Pipeline expansion Premier Christie Clark reached a novel revenue sharing agreement with Kinder Morgan (Canada) which will pay BC an estimated $1 billion over the term of that agreement. 

If Horgan and Weaver illegally try to obstruct the pipeline then I expect Kinder Morgan will claim fundamental breach of the agreement and pull back the revenue due BC.

See  Burnaby (City) v. Trans Mountain Pipeline ULC,  2017 BCCA 132  upholding the trial judge who said the province cannot interfere:

"Where valid provincial laws conflict with valid federal laws in addressing interprovincial undertakings, paramountcy dictates that the federal legal regime will govern.  The provincial law remains valid, but becomes inoperative where its application would frustrate the federal undertaking."