Usually these salts remain chemically stable for longer periods of time. In the case of alkaloids, they become protonated (here LSD-with-a-proton, a positively

Usually these salts remain chemically stable for longer periods of time. In the case of alkaloids, they become protonated (here LSD-with-a-proton, a positively charged particle; the tartrate being the negative counterion) and as such are thus less susceptible to oxidation. Can't steal negative charge from something that doesn't have it. And with LSD freebase that's not an unnecessary luxury, as it isn't the most stable against decomposition. To answer the question... probably this is the most stable formulation to get it into a pharmacy? I guess like you say, not blatantly calling it LSD helps image building too. With vyvanse, that's actually something else: not a salt but an amino acid covalently bound to amphetamine. This gives a much stronger bond than between tartrate and lsd ions in the salt, one that can be hydrolysed slowly in the gut to produce the active drug. LSD tartrate doesn't have a similar slow release property.