Because there's no real connection between Spurs and Bayern. They are not competing against each other regularly, especially in Europe. I don't speak on behalf of Spurs fans and neither do I speak on behalf of every Liverpool fan. We're not one homogenous group. As explained before Trent is a Liverpool lad born and bred - came through the ranks. It's particularly sore for some fans because of this and the way he's leaving without giving Liverpool any transfer fee for all the years the club have trained and given to him. From that POV it can be seen as why some fans have a grievance and don't like it and choose to Boo - they pay their money and have a right to. Said before Liverpool shouldn't have trusted him like they did with Mo and VVD who did sign. But football is a business, but also for fans it's about loyalty and the heart. The two things conflict.
See also why Owen is not treated with such reverence. We're not a cult, as you've previously said - as seen by Klopp leaving with grace and Slot coming and straight in. And it's not just Liverpool fans; do West Ham fans cheer Rice even though they got £100m for him, when they play Arsenal? It's just football.