Friend and I were having about an argument about whether the federal government could just blanket legalize something. For example, would the federal government be able to pass a law preventing states from making sports betting illegal? The way I understand it, this couldn't be done without amending the constitution. Am I right?
Yes, it can. Article VI of the Constitution makes federal laws supreme over state laws. This does not just protect federal restrictions of activities; it protects federal policy from state interference. If the federal government sets a valid federal policy that something should be allowed, this overrides all state laws to the contrary. If the federal government has validly regulated something so pervasively as to leave no room for any state involvement, all state laws on the subject are preempted.
For instance, there is pervasive federal regulation on topics relating to aviation, including on aircraft noise. Noise regulation is a traditional state and local power; it's certainly not generally unconstitutional for a city to pass noise restrictions. In the 1970s the city of Burbank enacted a noise restriction banning jet aircraft from taking off or landing at their airport between 11 PM and 7 AM. The only scheduled flight affected by this was a flight from Oakland to Burbank (i.e. intrastate). However, in City of Burbank v. Lockheed Air Terminal, the Supreme Court held that Burbank's ordinance was preempted by the Federal Aviation Act because the Act established total federal control over aircraft noise regulation. Congress meant to prevent states and their cities from regulating aircraft noise, and so they were forbidden from doing it.
For another example, Fidelity Federal S&L v. de la Cuesta was about state limits on mortgage due-on-sale provisions. Like with noise regulation, mortgage regulation is a traditional thing states can regulate. It's not a violation of the Dormant Commerce Clause for a state to set uniform rules about the terms of mortgages involving property in that state. In this case, California law had a rule restricting a mortgage lender's ability to immediately call in a loan when the property was sold to situations where the sale actually undermined the property's value as collateral. The Federal Home Loan Bank Board, however, had issued a regulation for federally-regulated S&Ls that said their use of due-on-sale clauses was to be governed exclusively by federal law, which imposed no such restriction. The Supreme Court held that the Board had been executing authority granted to it by Congress, and so the preemption stood.
In both cases, there were dissents. Neither case had a dissent arguing that the federal government could not preempt a more restrictive state law. At this point, there is no serious argument to be made that the federal government cannot block states from making more restrictive laws on something.
The federal government cannot command a state government to pass a law (New York v. United States, 505 US 144, Hodel v. Virginia Surface Mining & Reclamation Assn., 452 US 264, and this includes commanding a state to repeal or modify an existing law. It can, under the Commerce Clause, pass laws prohibiting actions (as long as the Commerce Clause would be applicable), whereby actions legal at the state level become illegal at the federal level. However, no government can pass a law making an act legal, because the default state is that all acts are legal, unless specifically made illegal. When people talk of making something legal, they are referring to eliminating an existing prohibition.
However, the states do not have authority to regulate interstate commerce – only the federal government has that authority, see Gibbons v. Ogden, 22 U.S. 1. So a state could not pass a law regulating interstate sports gambling. It could, however, pass a law regulating all sports gambling within the state.