They cause collateral damage through explosion. Stop the explosion is first low hanging fruit. Stop overcharging is also low hanging fruit. Common sense, nothing to do with free speech.
Not speech in this case, but freedom of action is being targeted, by the likely demand that people be prevented from making their own conversions.
Freedom, or the lack of it, is not just about speech and opinions being prohibited. It is also about whether the state interferes with what you are allowed to do, in the way of ordinary things, like getting out your tools to convert your bike with cost effective parts into a pedelec.
I have seen a video on saneagle's youtube site where he had made a conversion about a decade or more ago, with an electrically powered crank. He had fabricated a bracket and attached an electric motor. It was not a kit. From a quick look, it had certain safety implications, to do with an exposed, powered chain, and unless I am mistaken, pedals that rotated under motor power and might have conceivably smacked him on the ankles. This contraption looked well made to me, even if it did have the odd foible that might catch out the unwary. He isn't unwary though, and needs to be free to experiment and to make things for himself.
This principle should apply to all of us and not be taken away because one ebike in over three thousand when abused horribly by a careless fool, might catch fire. This of course would not be an issue had not the people abusing them been doing so in their houses - usually at night.
Oh and by the way, many of the ebike fires I have seen, were delivery bikes, blowing up in the street - maybe something to do with running two kilowatt motors from home made 56 volt batteries dangling about, unprotected from wear and tear and being bashed, in fabric triangle bags. These are often being fast charged and worked to the limit. If these monstrosities were out of the equation I think we might have a very much smaller number of fires than we have.