I opened a checking account in California and had the option to link my SSN (Social Security Number) to my bank account. The banker refused to tell me the pros and cons of linking my SSN to my bank account, on the grounds that it would constitute legal advice. Is that so?
Financial institutions in the US are subject to regulations that restrict what sorts of things non-licensed employees can talk about with clients and advice they can give about structuring accounts and payments in ways that might avoid triggering money laundering alarms.
I think this employee was being cautious about getting into a gray area and phrased the reason they couldn't talk about it poorly. The reason they were restricted from giving you an answer could be a legality, but not necessarily because they are giving you legal advice.
Probably not legal advice. More likely any answer to your question could lead into an explanation of tax consequences, which tend to depend on specifics of your tax situation.
Giving tax advice in itself cannot violate a law the way unlicensed dispensation of legal advice can. However, one can in theory incur liability for improper tax advice. Also, CPAs and individuals qualified to prepare tax returns are subject to IRS rules on giving tax advice.
Anyone can provide legal and tax information (as opposed to advice), but those who are close to professionals that can be sanctioned or sued for malpractice tend to err on the side of not doing so outside of a proper client relationship.
So back to your original question: A bank can (and I would argue should) communicate the general consequences of linking a SSN to an account. (In fact, it's not hard to imagine the FTC, OCC, or some other regulator at some point requiring them to disclose how they use your SSN.)
"Pros and cons" means you are asking for an opinion to help you come to a decision. The banker has no basis for understanding your circumstance. He wants to be your service provider once you have decided on the specifics of the service; he doesn't want to be your agent to decide those specifics. What if he misunderstood your circumstances and misled you? If one of your "pros" would be to make tracking cash flows harder (if you are say Denny Hastert) then that gets into legal advice. Does your banker know if you are embroiled in a hush money scandal? If not, this isnt a "pro" and there probably wouldn't say it, misleading Mr Speaker!
If you ask for the implications or next steps or what it means for specific IRS forms should you choose no SSN, do they answer your direct question?