Right. When he does it, it's so "obviously" a joke. And yet something tells me he would have no problem accepting help from anyone, legally or otherwise, if he thought he wouldn't get caught. I submit that maybe there are some things that presidential candidates should not even joke about, because even the whiff of impropriety should be anathema to decent, serious people.
This is at the level of people who claim they saw Sarah Palin say "I can see Russia from my house". If it really happened, and "we all saw it", it should be trivial for you to produce a video of Trump asking Russia to hack the Democrats.