First, they were not far from the eyes of the Qing.

Second, the vast majority of Chinese labourers in the United States, and that's what most Chinese there at the time were, were not immigrants. They were migrant workers. The majority would go to the US for a year or more, but then return to the Pearl River Delta after that.

Also, even if they were far from the Qing Court's influence/judgement at the time they were in the US (they werent), that was generally temporary, and for those who did end up staying, it was not usually the intention at the start of their journey. As those who stayed had children people slowly assimilated to a greater degree, as happens in most diaspora communities. But for the generation born in China, return was the norm, not the exception.

As to why it's not the case that they were far form the eyes of the Qing: Throughout places like San Francisco and really any city in the world that has something like a Chinatown, there are what welfare associations, most often based on town of origin. These are everywhere. If you live in a city with a Chinatown, you can walk down whatever main street runs through it and you'll see signs consisting of a few Chinese characters, often ending with 會館 or 同鄉會 or just 會. The characters which precede that are generally place names in China. In other places, associations were based on trade. In others they were more general. These are small scale, but in some places, California being the typical example, such associations merged. So in SF's history you hear about the Six Companies, or the Chinese Consolidated Benevolent Association. These are two names for the same thing, which is a merger of companies that has branches all over now.

These associations still had ties with their counterparts in China, and in particular with the Six Companies in San Francisco, operated as unofficial embassies. The Qing government was in constant communication with the Six Companies and the Six Companies would represent the will of the Qing government on their behalf. China didn't do too much to help with diaspora communities compared to what people might expect based on how some modern nations assist expats, but also the reason people were leaving China in the 1800s to find work was because the Qing was in no shape to be offering such large scale support in the first place, and had numerous famines, revolts, civil wars and invasions to deal with.

So anyway, thanks to the presence of things like the benevolence associations, people in California were not beyond the watchful eye of the Qing. Or of their relatives back home who would not have approved.

But also, and this isn't really asked, you'd also have to assume people wanted to remove the queue anyway, and for the most part there's just not much indication that that's the case. Regardless of peoples feeling toward the legitimacy of the Manchu Qing rulers in the 19th century, things like the queue were as much a part of their national identify as their language was, and social pressure would have also likely prevented a sole individual from removing it even if all the other factors I described weren't there.

tl;dr:

1. They werent beyond the Qing governments gaze

2. They were mostly only in the US and Canada temporarily for a few years to work

3. There's not much indication anyone wanted to remove their queues at that time anyway, even if there weren't a dozen other reasons not to do so, and anyway, it was mandatory to have one if they had any plans of going to China again.