It is of course completely legal for the US to apply any kind of economic sanctions they desire against any Chinese company, like Huawei or any others.
Nevertheless, it is stupid, for several reasons.
One is that there are no real reasons to apply sanctions to the companies selected until now and not apply the same sanctions to absolutely all Chinese companies. As the US themselves say, all Chinese companies must obey the Chinese government and they must assist their military if ordered to, so there is no base for banning commerce with only a selected list of companies, instead of banning all.
So the only reason why it can make sense to choose just some companies for the moment is that blocking completely all the commerce with China would hurt more the US than China.
However, for the Chinese companies it is clear that these arbitrary sanctions are not correlated with any behavior that they might try to have in order to be liked by the US in order to avoid sanctions, but as long as they remain Chinese companies they may be sanctioned too at any time. So they have a single exit, to never buy again anything from the US but substitute everything with alternatives.
Sooner or later they will succeed to do that and then the sanctions would not be efficient any more. The sanctions would have been successful only if there would have been 2 possible lines of action for the Chinese companies and it would have been certain that one direction implies sanctions and the other direction means no sanctions.
In that case maybe the US sanctions would have been useful. As it is, they can just annoy the Chinese and force them to give up on buying US products, which cannot be of any benefit for the US.
The second reason why the sanctions are stupid, is that they are hurting the Chinese only because they were sudden and unexpected so they have caught the Chinese off-guard, being still dependent of various components that include parts of US provenience. All this has happened only because the US propaganda has repeated during many decades that the global free commerce is good and that everybody should not waste time and money by producing goods that might be purchased from the US either at a lower price or at a higher quality.
Now all this formerly very successful US discourse has been exposed as a pack of lies, as it appears to have been just a trap conceived to ensnare other countries to become economically dependent on the US, in order to be vulnerable to economic sanctions.
Such a trap can work only once and the time and manner chosen by Trump were not at all the best. Now all countries will try to remove their dependencies on US and the next time when the US might need to have real leverage, for a better reason than now, it will be much less likely to have enough impact.