Actually you have that wrong. There is indeed a duty to mitigate damages in Wisconsin, and the statute that you cited does not change that. It merely clarifies what goes into computing what the landlord may win in action against a tenant that has abandoned the property before the lease ends. The Wisconsin Supreme Court, in a case that arose after the enactment of § 704.29 and that later goes on to discuss the application of that section on the issue of when the landlord has accepted surrender of the premises, clearly sets out the duty of mitigation:
Upon the tenant's surrender of the leased premises, the landlord has a duty to exercise reasonable diligence to re-rent the premises in order to mitigate damages. Sprecher v. Weston's Bar, Inc., 78 Wis.2d 26, 42, 253 N.W.2d 493 (1977); Patton v. Milwaukee Commercial Bank, 222 Wis. 167, 171, 268 N.W. 124 (1936); Strauss v. Turck, 197 Wis. 586, 587, 222 N.W. 811 (1929); sec. 704.29, Stats.
First Wisconsin Tr. Co. v. L. Wiemann Co., 93 Wis. 2d 258, 271, 286 N.W.2d 360, 366 (1980). So, there you have it, straight from the state's highest court: a landlord does have the duty to mitigate damages. The West Editors to the Wisconsin Statutes Annotated, explained the state of the law before enactment of § 704.29 and the reason for the Legislature adopting it as follows:
The Wisconsin Supreme Court is one of the growing number of modern courts which have announced that the landlord has an obligation to rerent when the tenant breaches the lease; this is merely an application of the fundamental damage rule that a party must use reasonable efforts to minimize his loss, whether in contract or in tort. Selts Investment Co. v. Promoters, etc., 197 Wis. 476, 222 N.W. 812 (1929); Strauss v. Turck, 197 Wis. 586, 222 N.W. 811 (1929); Patton v. Milwaukee Commercial Bank, 222 Wis. 167, 268 N.W. 124 (1936). Nevertheless the scope of the doctrine remains vague, and the case law dealing with acts which the landlord may take in mitigating damages is confused by tension with the old doctrine of surrender. It is therefore important to clarify both aspects of the law by statutory enactment, rather than leaving further development to slow and costly litigation. It is important that both parties to a lease know what the legal consequences of breach may be.
Wis. Stat. Ann. § 704.29 (West)(Editor's notes). As you can see, the duty to mitigate is an old one in Wisconsin, going back at least 90 years. The purpose of the statute was not to end that duty but to clarify some of the unresolved issues left out in the case law, like how the doctrine of surrender worked and what acts will constitute mitigation.
And a review of web sites by Wisconsin attorneys who practice landlord tenant law also state there is a duty to mitigate damages. For example, this comment from a Wisconsin attorney whose practice focuses on representing landlords on landlord-tenant matters states:
Put quite simply - a landlord cannot just sit back and do nothing and expect to collect rent from the departed tenant until the lease term ends. Per Sec. 704.29 of the Wisconsin Statutes, a landlord must make reasonable efforts to attempt to mitigate the tenant's damages by trying to re-rent the unit for the tenant.
From everything I see, therefore, it's very clear that there is indeed a duty on the landlord to mitigate damages. Starting with that very pointed statement from the Wisconsin Supreme Court.