Who are the they? Cory Gardner  endorsed Ted Cruz.  Because you endorse a candidate doesn't mean you owe a comment every time that candidate says something. 

Shall we interview Elizabeth Warren about Bill Clinton's  infidelity? Perhaps Warren owes us an explanation as to why Hillary keeps calling the investigation into her emails an "inquiry"

No that ISNT why. This has nothing to do with endorsements but the left  trying to demonstrate guilt by association. Something they often do to Republicans all the time. Simply because they are republicans,    but rarely if ever to their own.