Specifically, they regularly take an anti-scopolaminergic compound
to render their minds opaque to most known forms of telepathy.
If they didn't, then Ishan Anjar's jaunt to Betazed during the election campaign might not have been a good idea. When you're hiding a dark secret of that magnitude, a planet full of mind readers is somewhere to avoid... unless you're sure that no-one is going to be able to read you.
Think non-treasonous thoughts, think non-treasonous thoughts...
Although, it does make perfect sense that people would use these (pre-established) drugs. It must presumably be standard practice for prominent politicians and others in high-profile positions - or those with large amounts of sensitive information floating around in their heads - to take such precautions. Presumably Ishan reading as a blank to any probing telepaths didn't arouse suspicion on Betazed, and was considered normal - probably particularly predictable for a politician whose platform revolves around the need for tight security. There's nothing suspicious about it.
If not, then Cort Enaren pulled a Yoda on this one. "The dark side clouds everything, Supreme Chancellor. We have no idea where this Sith Lord is"