>"Using Android means no license fees, no restrictions on how you implement it (apart from those you impose on yourself and / or technical limitations)."

[IANAL]

If Microsoft's patents are not infringed or they are invalid, that is correct.

However, the former does not appear to be part of B&N's argument, while the latter is obviously subject to debate at this time.

Therefore, it appears to be premature to assume that this premise of your argument is well supported from a legal standpoint.