No one said anything about lying. And sure, if you've got tons of research offers, it's fine to say that you are premed. But if you have ONE offer and you have no reason that the PI should take you, potentially shooting yourself in the foot isn't something you want to do.
Nobody is saying that being premed is bad. The connotation that comes with being premed is bad and could hurt your chances at getting the position if there are literally hundreds of other candidates to choose from.
As an anecdote, one of my friends has been applying to labs since the end of his first year. He finally got one PI to reply back and scheduled an interview. The interview went great until the PI asked what his future goals were. He said he was premed and wanted to go to medical school. The PI literally said "Oh, you shouldn't have told me that" and he didn't get the position. This is obviously an extreme example, but there's really NOTHING that you can benefit from by saying you are premed in the beginning. Keep it vague, let your work ethic speak for itself, then it won't matter later on.