They can deny if they can prove you broke it, but they have to prove you broke it and they are not going to do that. I was a service tech for a long time and can tell you those stickers are not enforced. They are just there to keep people out that have no business being there. In the USA manufactured items with warranties are covered by an Act originally for automobiles but has been upheld many time since it's inception to apply to any manufactured consumer good. Usually if you get a company that is being defiant just let them know you are going to sue them in small claims court and ask them if the air line ticket and lawyer fees are worth their time to try and fight it? Unless it is a very expensive item. They will work with you once they know you are not going away.
Magnuson-Moss Warranty ActThe Magnuson-Moss Warranty Act is a United States federal law. Enacted in 1975, the federal statute governs warranties on consumer products. The law does not require any product to have a warranty, but if it does have a warranty, the warranty must comply with this law. The law was created to fix problems as a result of manufacturers using disclaimers on warranties in an unfair or misleading manner. More at Wikipedia
Long title:An Act to provide disclosure standards for written consumer product warranties against defect or malfunction, to define Federal content standards for such warranties, to amend the Federal Trade Commission Act in order to improve its consumer protection activities, An Act to provide minimum disclosure standards for written consumer product warranties, to define minimum Federal content standards for such warranties, to amend the Federal Trade Commission Act in order to improve its consumer protection activities, and for other purposes.
Nicknames:Consumer Product Warranty and Federal Trade Commission Improvement Act
Enacted by:the 93rd United States Congress