Yes, this is true, but this is true outside MTurk as well. If you rely on the YouGov panel (people complete surveys to get points for t-shirts), or really anything else, there's a strong incentive to cheat.
This is why good survey research is going to involve attempting to bound fraud through a variety of measures: attention checks ("What's your favorite color? Ignore this question and answer yellow."), looking for straight-lining (respondents always picking the leftmost answer), looking for unusual contradictions (i.e. asking the same question in two, opposite ways and looking for people who don't have the expected relation in their responses), looking at the distribution of completion speed and scrutinizing the lower quantiles, attempting to log participants who take the survey multiple times through dummy accounts, etc.
Oddly, my experience was that Turkers were fairly conscientious. I know one theory for this is that many Turkers are in fact Turking on the job, and so their reserve wages are not about what they are being paid for the MTurk hit, they're about what they're being paid to sit in their desk and not work.
I’ve noticed something similar to that in some of the YouGov surveys I’ve answered over the last decade. I wonder what it says about me that I sometimes write, in the errors/comments section at the end of the surveys, that I have seen surprising questions such as “what do you think about Channel 4?” when I have previously answered that I haven’t watched Channel 4 recently.
As you say though, it's the same problem with just about any study of this type whether MTurks, some other online panel, or students earning a little bit of beer money.