As a result of this, female members are under a constant barrage of mindless messages, which is what's being referred to as spam.
While this is true that some people subscribe to this logic[1], the number of messages (good or bad) that women get seems to out-weigh the number that the men get. I've heard (anecdata alert!) that some men get frustrated and will start sending out "form messages" because they will take the time to craft meaningful messages only to get no reply[2].
[1] I heard a similar story of someone (a "dad's friend" of a friend a long time ago) that would basically ask every woman at a party for oral sex. He would get lots of slaps in the face, but he never went home empty-handed (allegedly).
[2] I would personally liken it to investing time into job-hunting (i.e. trying to craft a decent cover letter) and then just getting no response from the potential employer. Though depending on the situation one other the other might be more stressful (e.g. searching for a job while unemployed).
This was documented on the old OKCupid blog. I think the post was taken down after Match bought them.
Men grow frustrated and get tired of wasting their time, so they stop sending in-depth messages. Women are then even less likely to respond, which ultimately sends men into a downward spiral of desperation.