This is very true. in most states the original purchase price is what your license fes are based on. And it never changes after original purchase. Buy the bike and get them to give you a percentage off any accessories you buy for a year. Then after the purchase is complete, buy the accessories.
The interest you would pay over the course of the loan is substantial, and the dealer does get a kickback, therefore they will try to convince you to finance.

By the way, If you walk in and plop thousands down in cash, they are required to report it to IRS. If I remember right it is anything over $2000.
Write them a check. That solves that issue.