That's not the way the checkAnswers button works (or at least has worked in the past to my knowledge). Either the checkAnswers button is displayed or the submitAnswers button is displayed, not both. E.g. when a prof is acting as a student (under normal defaults) the checkAnswers button is displayed but the submitAnswers button is NOT displayed. There were a few people who wanted both buttons (or just the submit button) in that case but I don't know if that was implemented.
The way WeBWorK used to work, guests could always check answers. Not sure when this changed but it was changed at least for WW 2.5.
I can see that a student could log in as a guest, do a similar problem and then log in as themselves and do their own problem. That's what I assume some prof's are afraid of. But prof's don't have to even allow guest logins (in fact most don't). I think the best option would be to have this be an optional course configuration thing and I would vote for the default being guests can check answers on open sets.