If such a bill passes as written, it removes any federal criminal prohibitions against marijuana (assuming their research was sufficiently thorough), and raises marijuana to the same legal status as celery salt, which simply is not mentioned by federal criminal law. There would be no federal declaration that marijuana is now legal, and therefore, when federal law is silent, state law can say something. If silence at the federal level precluded state law on the topic, there could be not any state law – the Supremacy Clause only addresses conflicts. The bill could also be written to include a positive declaration that there shall be no laws against marijuana, and that would create a conflict with state law.
Often, conflicts are avoided by crafting a jurisdictional difference, such as having laws that only apply while on federal property, or w.r.t. federal employees. The Constitution is structured to separate what the federal government can do versus what state governments can do, so that conflicts don't arise – however, the Commerce Clause ends up being used to expand federal power (e.g. the Controlled Substances Act). As Nate Eldredge comments, there is a huge difference between what is allowed under federal vs. (certain) state laws w.r.t. fireworks, and even town-to-town variations in fireworks laws. Numerous "professional" acts are completely legal w.r.t. federal law but are illegal at the state level. No federal law prohibits me from practicing as an attorney: it's state law that does that (likewise, being a cosmetologist or selling beer).