If they start advertising companies that don't succeed, the effectiveness of their advertising diminishes. Thus, there is monetary incentive for them to advertise as much as possible, but with discretion.
Really, changes in integrity should be pretty transparent. If their recommendations start failing, they are mis-evaluating or their analysts are lousy. If their picks continue to succeed, their core business is staying consistent.
Also, they don't just make recommendations. For paid subscribers they provide custom research reports, tips on short-term opportunities, daily monitoring of the market, and general investor insight. This market probably wasn't near as big as it is now due to the influx of online brokerage firms.
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