I know that. I do not understand what exactly happened.

For example, you say, "左侧的车在一路口左转,我就想这是一个机会,因转弯车辆会slow down后面的车,于是我就打灯准备换道. 没想到后面一辆车加速赶上来": how did that police car catch up? after the left-turn car turned? If yes, then seems to me, you do not really have time to switch lane. (BTW, you did assume other cars will slow down; for different reason than police said.)

And here, "我减速它就慢下来,我加快它就快,一直在我左后侧跟着,却不跟上前面的车. 这样试了几次,眼看离我要左转的路口越来越近,我将速度放慢,这样就与前一辆车有点距离,然后加速换到左边." which one is the "前一辆车"? The police car?  If yes, he did let you change lane; it is only you want to switch lane in front of it that caused the delay.  If not the police car, how could you achieve distance with a car in front of you by slowing down?

I rather believe you that the police is unfair.  However, from what you said, I could not get that conclusion. "Unsafe Lane Change" seems quite proper.