3D printer part clones from china - legality
This question is really a legal question, and could apply to any cloned parts/devices rather than being 3D printer specific, and a generic counterfeit consumer goods based question should be asked on SE.Law. However, as you rightly state, a lot of 3D printers from China may contain (whether knowingly sourced or not by the manufacturer1) counterfeit parts, be that ICs, hotend designs (i.e. E3D clones), controllers (i.e. Arduino Mega boards or stand-alone non-RAMPS Arduino shield boards),or what have you. As such this is an issue that may be faced by any unwittingly innocent consumer.

Prefacing any statement with I am not a lawyer (IANAL), these sections from Wikipedia entry on Counterfeit consumer goods might help answer your question:


Growing problem as will this 
Enforcement- US. 


In short, if they have a mind to, Customs could seize it at the border; if your house was raided (for whatever reason) law enforcement could seize your printer (although this seems unlikely, unless they were explicitly raiding your house for knock-offs); and (more worryingly) there is a proposal to fine those people who purchase knock-offs.

Of course this is not the only legal issue that may be encountered when buying Chinese devices/parts from less-than-reputable suppliers o eBay or AliExpress, for example, what if:


it breaks and what legal recourse do you have as a consumer; 
it produces a poor quality or dangerous print; 
it explodes, what legal recourse do you have; 
and so on, etc. 


These questions lead into rather murky grey and legally complex areas, and really would need to be dealt with by a legal professional (solicitor/lawyer).



Other Stack Exchange posts worth reading

These deal more with quality not legality but see 


Can cheap hotend parts sourced from China actually produce good prints?, and;  
Vaguely related - What are the pros and cons of collecting parts yourself, versus getting a DIY kit and then modifying it?
Also, which respect to the electronics: 


Some Arduino components seem ridiculously cheap and;
Compatibility between Arduino and Arduino clones.
This answer to Arduino Clone - A request for the USB device descriptor failed interestingly points out that if the device is licensed appropriately (CC, open-source, etc.) then they may not be any legal issues at all related to cloning (but not counterfeiting - See What's the difference between a clone and a counterfeit Arduino?.





1 A bone fide low end oscilloscope manufacturer got stung by a batch of fake regulator ICs just last year, see Re: JYE Tech DSO150 oscilloscope troubles